making a ceiling-tray thingy

Id appreciate advice on something I want to make for my dining room ceiling. I want to make something like an oversized oval serving tray, only the outside bottom and sides of which will be visible (painted a particular blue). This would be an oval 36 inches by 54 inches, with the sides about four inches high. The purpose of this is to hang three lightweight pendant lamps from it, so those can be centered over my dining table. The big oval is big enough to extend to an off-center ceiling electrical box, which cant easily be moved (concrete slab ceiling). Im an apartment dweller with only hand tools, a jigsaw, and a power drill. My tentative plans:
=Plot an oval on a computer plotter, mount that lightly with spray adhesive on the Masonite, and cut the oval out with a jigsaw, then rasp and sand that to be as close as possible to a perfect oval. I thought Masonite would be the easiest to make really smooth. Would thin plywood or drywall or something else be better?
=Put a 4-inch strip of veneer around the perimeter, gluing it to blocks spaced every few inches around the top inside perimeter of the Masonite. These would probably be dominoes or childs blocks from the dollar store (something thats already cut perfectly square). Where the veneer strip ends join, should I put an 8-inch piece of veneer on the back side across the joint (imagine it glue side to glue side, though veneer this wide may not come preglued) to make sure the curve is smooth and the ends dont separate? Seems like I should put a bead of glue all around the inside where the veneer meets the Masonite oval. How do I keep the glue from seeping through to the finished bottom side?
=Run three or four ribs across the oval, glued to the dominoes and to the Masonite. Then put a couple of stringers between each set of ribs, also glued to the Masonite. This framework is to keep the Masonite from sagging or corrugating. Should that be balsa or just regular 1x4s? Is there something simpler? I thought about using the aluminum Ts used for suspended ceilings, but couldnt think of how to attach the Masonite.
=Fill (very little, I hope) the corners where the veneer strip joins the Masonite, sand lightly, and paint the outside and drop the pendant lights on cords from this platform. Then put four lead anchors in the concrete ceiling and somehow suspend the platform (probably with squeeze-type gate latches).
Thoughts on how to make this easier or better?
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On Sat, 12 Jan 2008 21:02:17 -0600, Mr Downtown <"Mr Downtown"> wrote:

With my tools, I'd make what's called a "torsion box" from plywood.
With your tool selection, I might make this out of blue 2" thick foam insulation board, skinned with 1/4" cabinet grade baltic birch ply and 1/8" ply or plastic laminate (countertop stuff) edges.
The oval would be cut and shaped of foam, and 4" high 1/8" thick sides would leave 2" to hide the hardware. The 1/4" ply would be less likely to wrinkle or ripple than hardboard. A foam safe contact cement would attach the ply nicely. There would be scrap material available to practice on. The only pre-paint filling would be the joint where the 1/4" and 1/8" materials meet, and the seam in the 1/8" strip. Bondo would fill well.
The whole thing would be light and strong.
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On Sun, 13 Jan 2008 02:55:21 -0500, "Bonehenge (B A R R Y)"

I forgot to mention that the smooth foam edge would probably help the edge stay smooth longer. 2" thick foam gives lots of edge for gluing surface.
Over seasons, an edge attached to scattered points might develop flat spots, wrinkles, and buckling.
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Bonehenge (B A R R Y) wrote:

Thanks. I like the idea of the foam rather than ribs and stringers. Can 1/8" ply (is that plywood or something else?) be curved that tightly just by hand? Wouldn't it fight pretty hard to separate from the foam? Or would you somehow join the ends together with something on the inside? Why are you suggesting 1/8" ply rather than just a strip of veneer? Any thoughts on how to nicely cut four-inch strips of plywood with no table saw?
I presume you'd glue the plywood to the foam before cutting the ellipse. How would I smooth out imperfections in the curve? Also seems like it would be hard to keep the sides of the foam exactly vertical using a hand-held jigsaw. Maybe that's why 1/8" plywood rather than veneer?
How would you attach it to the ceiling? 1/4" plywood seems too shallow to take eyehooks. Would I cut four holes in the foam and glue thicker blocks in there?
I was being imprecise when I said oval. I actually meant an ellipse, or possibly a superegg.
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On Sun, 13 Jan 2008 16:56:38 -0600, Mr Downtown <"Mr Downtown"> wrote:

Yup. You can also wet it or steam it to help.

Less "wiggles". The 1/8" ply will fight the bend enough to stay smooth. I would think up a design feature that incorporates the joint, rather than trying to completely hide it. Google Images "Shaker Box".

A sharp craft knife (Xacto) knife and steel straight edge can cut 1/8" ply easily. I did it for years on large scale model aircraft. You'll make several passes with the knife.

Use a bread knife or coping saw, smoothed with a sanding block and 80- grit paper. I'd use the perfected foam as a pattern for the 1/4" ply. Cut the ply _slightly_ oversize, glue it,and sand to the foam.

That would probably work well. Remember, this thing won't weigh a lot.

Foam is very easy to work and form. <G>
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Somebody wrote:

Yes, especially if you use bending ply.
Lew
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(snip)
You don't need a computer to make a nice oval. Two nails, a loop of string and a pencil will do it.
Drive the nails in along the centre line of your oval evenly spaced apart, then place a loop of string around the nails place the pencil in the loop and draw the oval. The shape of the oval is determined by the the distance apart the nails are (closer together more like a circle, further apart opposite)the size of the oval is determined by the length of the string.
Mekon
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