For core material, you may want to consider this; Macrocore 33
I used a small piece on a small deck recore, and the US Navy is using it in the DDG1000...
I found that it is light, rigid, and it bonds really well with resin. Also, it will NEVER rot.
If you are interested, PM me, and I'll connect you with the right people.
Note that I have no interest in Esyntactic, other than I know the owner.