How to reinforce cross-grain

I already know this was an ill-conceived project, but I am too far into it to abandon it.
I found a pile of pretty nice 2x6 PT abandoned in the street. I planed them, routed a glue joint edge, and glued them together. My plan was to then paint them with truck bed liner (polyurethane) paint, put some foam floatation under them, and use them as a dive platform. The glue seems to be strong enough, but they flex pretty bad (whole thing is about 4' x 6') so i also threw in a bunch of pocket screws. Didn't help much; so I am thinking about painting it, and then putting on two aluminum angle irons. If I screw the center down securely, and use slots up to 1/2" long at the ends, will that accomodate the expansion? The angle irons will make it easier to attach the foam, so maybe I can justify it that way.
Any other ideas, other than throwing the whole thing out and using plywood like I planned before finding the PT?
Thanks. I hope you could make some sense out of my jibberish.
BTW, I glued up some boards and anchored them in the lake last October; just to see if the glue would hold together. They seem no worse for wear; though they aren't floating very high!
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I'm thinking this is floating in the water?? If so, you won't have to worry about expansion /contraction--- WAIT, you said you were going to add foam-- SO, it will be above water & will get wet ONLY when you dive from it (or it rains) . The angle is a good idea. If you want to make it sturdier, you can screw some 2 x 4's or 2 x 6's to the bottom in addition-- plut in slots for the expansion & contraction also. BTW-- the plywood ( I think) is not a great idea- Even PT will delaminate faster than the 2 x wood you have already spent time on. What kind of glue did you use? Also, you can use the 'great stuff'-- or other brand-- foam in a can to glue your foam sheet to the underside
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I used titebondII. It seems to have held up.
You might be on to an idea though. If I put it in the water, it should absorb a lot of water. If I then let it air dry for a few days and coat it with the polyurethane paint, it should expand as much as it is going to, and be pretty stable. I can then attach the angle iron directly, without long holes. It will be too cold in the winter to drive the moisture out, and too humid in the summer. But there might be a problem with paint adhesion. Hmm.
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