Movement and Miters

I'm planning on gluing 4 3"x3" doug fir boards, mitered at 45 deg., to 'frame' a plywood box that is 72"x45"x3". I want to allow for a 50% humidity increase (inland California spring to coastal Texas summer) (30% to 90%). I'm assuming the ply box doesn't move, and I can neglect the lengthwise movement of the boards as they will have a nice large glue area to the ply box ( 216 and 135 square inches for long and short sides, respectively). Not sure if the radial movement will give me a problem or not; assuming boards are plainsawn.
My concern is the miters. The boards may expand as much as .12" at right angles to the box unrestrained, which means a potential .17" gap in the outer edge of the miter. Yuck! What kind of joinery, at a minimum, am I going to have to employ to stop this? Or, can it not be stopped with most practical methods? I'm assuming that gluing up the miters and popping in a couple of dowels won't cut it.
-Chance
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I'd probably go with a mitered lap joint. See fig 1 on the top of page 2. http://www.practical-sailor.com/newspics/charts/878hatchscreens.pdf
Art

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I've been fighting this for years. My home is 70 feet from the ocean and I see high humidity in the summer (150%) and 0% in the winter. In my experience there is nothing that will permit a miter joint to look good all year 'round. I'm about to settle on all lap joints.
RB
Chance Casey wrote:

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