I normally use comstock traps for squirrels and raccoons. I have trouble mounting them sometimes to the side of a structure (if the hole is up off the ground). I was just wondering if anyone could give me some ideas or pictures of how to do this better. I normally just use screws with washers, or metal straps on them to mount straight to the structure.

Just add flanges to the trap. Even a nose cone with flanges on a 12 x12 x 30 can be attached to a gable vent if you need to cage a raccoon alive.

Nose cone and lots of strong zip ties for a raccoon.

A few zip ties to attach to cage. You'll see a bungee too. She popped that off, but it was plenty strong enough. Had to catch her alive, the pups were behind a horse hair plaster wall in this 1800's building. I scared the crap out of her and released on site, she moved the pups that night out of the house.

Most of the time I will cut a board to go over the hole. Securely attach the board over it and then attach trap to the board. This makes switching the trap out pretty quick and easy.

I use many of the methods mentioned here with some of my own addition s.
I carry and use an assortment of boards to block off and reshape an irregular hole to transition into a trap.
I mount the board as a secure base for the trap.
I add boards for blocking as needed.
Carry an assortment of star drive construction screws to stack boards.
As Joey shows, use hex head screws for mounting wherever possible. Fewer stripped and dropped screws.
Carry a spool of electric fence wire and heavy duty zip ties because they are so universal.
Medium duty zip ties are great for fastening nose cones or flanges or panels to a trap.
I carry some folded shapes and sizes of hardware cloth to fill in around a trap or close off an opening.
I also use a couple types of nosecones plus carry both 1/4” and 1/2” hardware cloth to make a specialty nose cone. i use a short piece of electric fence wire to attach the nosecone so it does not slide off in transit or while walking.

I started dropping a lot less screws when I switched to hex heads and nut drivers. The magnet makes it way easier to put screws in one handed. I pretty much only use 1”-2.5” roofing screws. Use to use lathe screws that are philips heads and I don’t even touch them anymore.