Hi Jim,
First, I would make sure the wood is completely smooth and has a waxy surface. I’m not a woodworker, so I’m not exactly sure what you would need to do to get that finish.
As for a mold release, the tricky part is that your wooden box isn’t flexible. That means you are going to have to find a way to lift out your casting. For me, I would try using wax paper as the liner as I’ve had good results with resin not sticking to wax paper.