Before this week's meeting, there was a discussion about material that could be used for a battery case on an electric bike. I suggested bamboo for cost, weight, strength, (and easier to work with the tools we have than aluminum most likely).
Here's a source I found locally that has a 0.71in thick piece that's 18in wide by 24 in long (the have longer if needed):
That might be thicker than you want, but a layer could be milled off (or re-sawn then sanded).
I have a membership and can bring a guest if anybody wants to check it out and isn't a member...