Began by measuring a cut line along the aft edge of the cowling based on the lines I drew on the inside of the cowling using the sharpie-on-a-stick technique up through the cowling space around the nosewheel. I chose 30mm overlap between the aft ends of the 2 cowl pieces and the forward edges of the fuselage skins with the attachment line 15mm from the aft edge of the cowlings. I cut off the excess using a dremel cutting wheel, created a huge dust cloud in the garage, and got both pieces trimmed down to their final size.
Once trimmed, I placed the two cowling pieces back on the airplane and attached them to each other with clecoes using the holes I drilled yesterday. Once assembled and placed into the desired position (and held there firmly with duct tape!), I drilled out the 5 holes across the top, three holes up each side, and the final hole at the aft end of the seam between the 2 pieces as directed by the plans. I'm pretty happy with the final fit. I still have some sanding to do along the aft edge and a little grinding to even uut the line where the top and bottome cowls meet in the back.
Final trimmed size with pilot holes where the cowling attaches to top and side skins.
From behind: Another view of the trimmed and clecoed cowling.
Prop spinner mounting plate spacing still good at 3/16' to 1/4" (must be no closer than 1/8").