Speaking from lots of experience with trapping those nasty EPM makers, if your possum likes cat food go with it, with a little bit of other yummy’s on it. Put a small amounts.
Lock the cats up completely until mr. epm is caught. Cats may go in the trap but they will never again. If mr. epm is regularly dining at your table, put food in the trap, and in the trap only. I would put an extra tidbit of tempting yummyness on top of the cat food. Like a piece of chicken or turkey, cooked, like the skin, or a neck, or a wing tip, fried is good or boiled is good - aromatic is good. I have also used leftover stuffing/dressing. I leave a little few tidbits of dried cat food, and the yummy bits you have for it - 3" before the cage entrance, and a little superinformationhighway trail of 2-3 tidbits of the yummy food down the cage to the metal plate and then on the other side of it do I leave all the rest of the food, not alot but just enough. Center the food between the plate and the edge of the cage. You do not want access to the food from outside the cage. I would put it on a piece of paper inside the cage. Maybe the inside of a feed bag. But it is not necessary to do this.
Once you have said mr. epm in the cage, take the cage and the possum, and take it to “the place” and shoot it IN the cage. Take alot of bullets with you because they take many many to do the deed. So be prepared. Clean all possible epm poop and put in bags and toss in the trash. Take the cage and the body to a designated 100% guarantee the buzzards will find the body or place body in bag and put in the trash. You want 0% of the body or entrails left on your property where horses may be. Rinse cage really really well away from where any horses are or will come in contact with the poo.
They are so nasty to look at. Ick! They hiss, and drip saliva from their teeth, and their ears turn pink when they are angry.
I just shot and killed a skunk in the trap last saturday. It was a female. Buzzards ate it. No more babies from that skunk, and now all is quiet on the farm front, and no more smell or poop on the driveway. My barn is safe, for now. Skunks are the worst to deal with. They spray no matter where the bullet is placed. Epm maker, no problem, easy!
Good luck!