Thanks for the replies so far. So with the suggestions, it would be better for build a box type fuse and sand it to fit. I can do that as well. I will then go with one pair of spruce 3/8 square stock for the length of the fuse, along the thrust line. This will also be the support for the horizontal stab. Then I will use 1/4 balsa on the sides and sand to shape.

Here is an autocad drawing of the firewall with three views, one is the firewall with the balsa sheeting, the second is the diagram for the cutting, (should this be 3/8" ply?), and the third is what the final shape should be.

Thanks for the help so far.