For the firewall i need something that's 4mm or 1/6" thick, so i don't have to modify other parts. Do you think CF laminated 4mm cross grain balsa will work or is it too fragile? Since i'm going to lasercut another firewall, i can test a few ideas.
I have used CF laminated end grain 4mm balsa as a firewall in the past for 160 2-stroke F3A motors-the only addition was to anchor the captive nuts, for the motor mount, in separate little doughnuts of liteply glued to the rear face, to avoid piercing the CF skin.
200 gram cloth, or even 90 gram, works fine.
Nowadays, I use two layers of 2mm liteply, bonded together with the outer grain of the two layers at 90 degrees to each other;
and faced with any choice of 200 gram glasscloth, 200 or 90 gram Carbon fibre cloth, whichever is available.
This firewall has worked for me, for electric, 2-stroke, 4-stroke, without problems.