One way...
To make a consistent gap between along cowl edges, I used a strip of, say, .032 aluminum and stuck the appropriate grit of sand paper with spray adhesive.
I have also sprayed adhesive on the back of sandpaper and place several popsicle sticks on it. Then use an ex-acto knife to cut them out. Sorta makes a bunch of one-sided fingernail files. Worked good for me. Good luck.