that thing is amazing looking, took minutes to get over how good it looked just to look at what you built lolAlmost finished with this one..... albeit only a big tongue box..... but any composite with a medium in center will make for a great sammich board.
I would use 3/4 or 1 inch polyurethane foam board. You can pillage them at any hardware store. Either pink or blue.
Polystyrene us too susceptible to chemicals....IMHO.... carbon fiber is pricey, but glass and a good epoxy will be fine.
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Been doing a little reading on aluminum and a little less reading on the sandwich board approach. There seems to ver a dizzying array of options with the boards. To the point it's hard to understand what works with what. Haven't had much time the last few days but by the weekend I should be back on my research.
For that box what materials did you use? did you make your own sandwich board or use premade panels?