The correct product to use to support the boards you plan on using to support the bed would be the e track wood beam socket 59149.
E track bunk beds.
Its designed to have 2x4 or 2x6 boards nestled in them to keep your cargo from sliding around in a trailer.
I would recommend using two etracks that will be on either side of the trailer interior walls running horizontally at the height that you want the bed to be.
I have had way too many beers to remember if i came up with this idea or if i saw it somewhere else so im not going to take credit for it fully.
This single 4 foot long section is rated for 6 000 pounds of course the way it is mounted and what it is mounted to also would have to be rated that high to get the full capacity.
E track custom hammock beds for enclosed trailer.
I bought 4 2x4x8 s and routered the edges down smooth to keep any wood snags and bumped elbows at bay.
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Since you will want to have the 2 by 4s on edge to support the plywood and the only way the 59149 could be installed in this fashion is if the e tracks are installed horizontally on your trailer walls.