When using them, cover the pan and fit just inside jaws and not over jaws. I don't use them though.
I have been using polly fill pillow stuffing under the pan also. It has worked fine and I have been mostly happy with this.
A friend and I just started working together setting on a large hunting lease. For the first time yesterday, I used pre sifted peat moss to fill inside of jaws. You don't need pan covers or polly fill with the peat moss. I did like how easy it was to use, and also left me with plenty of dirt for sifting and blending the set. Also, the dirt is wet from all the rain, I was glad to be putting dry material inside jaws to maybe help keep traps from freezing with the coming cold weather. After jaws are filled with peat, sift a thin layer of the dirt back over everything. Then blend if needed for flat sets.
No catches this morning, but the only dirt hole we baited with bacon grease was dug. Remade with some better guiding and backing, and they will probably be back for more bacon!