advice on re-core fuel cell hatch

[phatdaddy]

i had a fun day today, actually not to bad, thanks to some great advice from vic, did this in 1.5 hour with a $4.00 flapper wheel from harbor freight.



was going to use foam like nidacore or divinicyl, but cant find it locally in less than a whole sheet. probably going to go back with plywood. whats a good one to use? the wood in there was 3/8, thinking about going back with 1/2 or 5/8.

Ideas?