Here's how I'm doing the bottom cowl.

I added an inside lip to each side with a mount with a 4-40 blind nut.

In the cowl I took 2 1/16" ply pieces and made into a 1/8" insert. Between the ply pieces I added a 4-40 washer to keep the ply from wearing. The cowl siding is 1/8".

Later I'll add a 3rd bolt to the front center after shaping and sanding the front end and after I've cut out my air intake for the motor.