The cowl in the pic was built up from balsa.

First, square up the firewall to what standard you want. I use zero right thrust & ~ 1/2 of the stock down thrust if I'm removing the dihedral from the wing. I use balsa wedges to square the firewall, then cap the balsa with 1/8 ply. Cut out the fuel access hole.

Re-mount the engine, centered on the firewall (upright or sidewinder -- whatever you like) & remove the carburator.

For the cowl nose-piece, I start with a 3/4" thick balsa block, cross laminated from three layers of 1/4" sheet. I cut it to fit the max outside dimensions of the fuse/nose, then hole-saw the port in the center of the raw block for the engine crankshaft. Tack the block to the backplate of a plastic spinner with CA & little 3/32 balsa stand-offs. Mount the block on the engine using the spinner back plate to hold it in place. You may have to trim the ends of the "cowl cheeks" to get enough clearance to mount the block onto the engine.

Next, using 1/4 thick balsa sheet, I join the block to the fuse nose, filling the gaps at the sides, top & bottom.

Cut through the tack-blocks holding the nose-piece to the spinner & remove the spinner & engine from the plane.

Sand & file the nose block to shape, along with the balsa-sheet filler pieces. Fill irregularities with microballoons.

Grind a recess on the back of the balsa nose piece for the carb & do trial fits until the engine will mount with the carb in place.

Finish-sand the external surfaces & grind away excess material inside the engine compartment with a dremel. Paint the engine compartment with rustoleum or whatever fuel proof paint that you like.

Touch up the external covering.

Done.

The whole business can be done in a couple of evenings