By the way, the design for ours will need to span across the aisle to make the sleeping platform. With the baltic birch only method, the plan was to have a panel/s that is held to one side by piano hinge and then flips over the span to rest on a support rail. With the road case method, I've yet to come up with something to do this. Have you seen anything, HMR ?
Yes. One of our original sketches used a similar platform for the bed. I don't know how big of a space you need to span but if the width is, say, 3' or less, I would use 1/2" Baltic Birch (see note #1 below), laminate both sides with ABS and use 1/2" "U-Cap" extrusion for the edges. On the edge with the piano hinge (see note #2), I'd drill completely through the wood for each hole (you could also do every other hole depending on piano hinge) and use #8-32 x 1/2", stainless steel, pan head screws with a matching T-nut on the other side. With a little loc-tite on each screw it should withstand many miles of washboard roads. Also be careful when tightening any fastener on top of the ABS. The plastic will discolor and warp if it gets squeezed too hard.
Note#1- I purchased our plywood from a specialty shop so I could get a "true" 1/2" thickness. Home Depot and Lowe's usually sell their ply in 15/32" thickness. When laminating both sides of a sheet with ABS it's probably better to use the 15/32". The extrusions can be a tight fit when trying to overlap two sheets of ABS and a 1/2" thick ply. It's always good to experiment with a little section, first, to ensure the right fit.
Note#2- Just a reminder that there are a ton of different piano hinge sizes/configs available. You don't have to stick with the standard width offered by the road case suppliers. The standard size will work great for what you want to do but you might find something else that works better.
Aircraft Spruce has a nice selection of piano hinges.
Finally, we ditched the idea of the plywood bed base when my wife suggested an Ikea slat frame. This was probably the single best design change we made. If there's anyway to fit this into your project, DO IT! I can't express how much more comfy this setup is.
The frame: