What I did for my Strikes and Spares, after testing the inserts to see if they'd fit, was to raise the playfield and use thin folded strips of sandpaper(I just cut them with scissors). I'd stick it through, grab it with both hands (one on the top side and one on the bottom side), and use a quick filing motion. I just stood on the side of the playfield and reached around it. It worked perfectly and quickly.
I tried files and picks and blades, but this worked best.
I used 14 stars to do 10 correctly. Sometimes you can't tell for sure if they're right until you've actually installed them.