Thanks for this idea of a sandwiched panel. I was going to use an aluminum ramp for the span with plywood on top, but this sounds so much better.
I used Reese bifold atv ramps for my overhead shelves as well as for the shelf/bench/sofa/double bed in my van.
The neat thing about the ramp sections is all the features and the engineering to minimize weight. Welded laddered structures are very strong. A 12.5 lb 15"x72" section is rated for 650 lbs.
The raised lips were intended to hold wooden or other inserts when needed.
The slip pin hinges lock securely when opened and require no tools to slide apart when folded.
I actually used two sections to drive a community snowblower into the van the other week---closest I've come to atv use!
This suited my needs for a lengthwise bed and my need for a clear center aisle to stack canoes otherwise. I use the sections with or without bench cushions. The 1/4" ply on the cushion bottoms only has to span a foot in either direction---but it is strong enough to extend the ramps a few more inches to accommodate 6'4" me.
Back to the original intent of using sandwiches to minimize height and weight:
You might look at using a hexagonal panel material for the best strength/weight combo. I saw 24x60 aluminum honeycomb panels on ebay but did not look into gauge or ratings.
Companies like Hexcel make (likely expensive) panels with choice of many materials.
And one last aside about glueing ply to al---are the differing expansion coefficients a problem for a 60" span? I've always just floated my ply in the al channels and never considered fastening them.
A marine specialty plywood distributor was selling sheets of foam in various thickness with ply skins on both sides when I built my DIY. Perhaps a good material to float inside an AL frame?
Sorry for too many ideas in different directions.
Dan