I ended up using 3/16" cheek doublers instead of the 3/8" ones that came with the kit, and I used 3/8" (I think) tri-stock. I took the fuse along to my daughter's softball practice tonight and sanded for an hour in the parking lot. Here it is before and after sanding, and with the engine mounted.

I still have some more shaping to do, but it's close.