Moesha, get a few older towels and/or a blanket or two you don't really like, and put them flat and stacked on the carpet.
Put the trap on top...any pee will soak thru but not to the carpet.
Keep the trap covered...he'll be less likely to panic that way.
I like the trash bag idea...but any pee will run off the plastic and onto the carpet; that's my concern.
Good luck at the vet tomorrow. :)