what size/thickness/mesh opening do you use?

I use aluminum window screen, but regular window screen works too.

Ok total beginner question here but what is the purpose of the pan cover screen? I've seen it done on videos but never heard it explained.

It keeps dirt from getting under the trap pan.

In early Fall when ground is fairly dry a pan cover can be omitted. Rain, birds, and mice can cause dirt, trash, pebbles to get under pan and prevent trap firing. I use aluminum screen pan covers. Most trappers cut pan cover to fit just inside trap jaws. The aluminum screen is not stiff enough to hold much weight when cut that way. Relying on screen flexibility, I cut my pan covers large enough to cover trap jaws. Then whether I use dirt or just a few grass strands to cover or disguise trap, the covering is more durable. With an animal's foot on the pan that flexible screen will easily bend out of the way and is too short to be caught in the jaws.

I use coffee filters. Cheap and easy to carry

I'am going to try landscape fabric this year, last year I used fold top sandwich bags they worked ok, some times they can draw moisture under the trap

I use nylon screen I scrounged from a hardware store that does repairs. I don't like Aluminum because it's too stiff, and might jam the jaws if caught, nylon will give.

I run MB550s and KO K9 Extremes. Both of these traps have a pan that sits low. I cut my aluminum screen to go under the jaws. No worry of it getting caught between them. I use grass or peat moss to cover my traps, and the aluminum screen is my favorite.

I never thought of it that way, but that makes sense. I'm going to try that this year. Thanks!

You are welcome. I just let it go 1/16th to 1/18th inch past outside jaw edge. Last season I trapped coyote and cats with just a few blades of grass covering. That worked better than dirt covering in the freeze/thaw conditions I trapped in. I don't care to lug around dry or waxed dirt. Calcium chloride leaves a wet appearance that cats, and probably some coyote, shy away from.

Coffee filters or wax paper precut.

I like the aluminum screen idea has anybody ever used the pieces of foam that are put underneath the pan?

I have and I don't like it. To each his own.

Yep for jaw spread traps 6 and over dogless, Foe 5.5 I use aluminum screen. Works best when cut so the corners under the jaws to lock it in place. The keg creek screen works real well with dogless #3s

And Victor Monty 4s
Screen is made for a dogless trap, and works well with a dog on trap also.
Above is a premium product.

I suggest testing pan covers by firing a trap with a stick.

I use single ply unscented toilet paper. Cheap and multi purpose...lol

I've never tried screen (might this year), but I use and like poly fill. Cheap quick and easier than the pipe insulation (foam).