Matt, I ended up using two pieces of 3/4 plywood (treated with marine epoxy with added aluminum powder to give it some colour, yes I am that crazy) per cavity, joined with some aircraft floor rail extrusion down the centre line. The extrusion is shaped like a very squat I or a very tall narrow H depending on your view. It fits the 3/4 ply really well and is the top of the I is probably about an 1/8". There is also a good deal of pink styrofoam around the tanks for a bit of insulation but mainly to keep it all snug yet not too rigid. At the time I had wanted to use Teklam sheet but it would have cost $1100 per cavity so I economized.
1960 Sovereign 33' Pacific Railroad Custom