I want to get rid of the canvas support in cabin. I have 1/4 inch ply cut to fit. Am I able to fiberglass both sides and make it strong enough for 2 to sleep on? If so, how many layers and what material do I need to use? Want to keep it light.

Id beef it up to 1/2 or 5/8 inch and build a center support off the two stringers and screw it into place that way you still have access to underneath

Was planning on cutting an opening over where ac unit goes and attaching piano hing for access there.

Trying to figure out if and how to make the 1/4 inch strong enough without a bunch of supports.

Pack when we had our gun made pops placed 1/4” wood right over the top of that sail loft thing that supported the original cushion. It worked out well.

If you want light, I would run some aluminum angle crosswise to support it. I think 3 pieces should be sufficient. If you bolt it to the side supports you can always remove it for access.

a "T" section of aluminum glued , screwed and glassed on to the panel.