It's tip-board time.
PB at end of a board that tips into a (tall) bucket with the rat's weight. Your choice, dry if you want him to live and be relocated, wet (water or oil) if you wish him to drown.
If you hinge it to the bucket, it 'resets' iteself once the first rat falls. 1x4 s4s pine works well, claws can't get purchase as the critter slides in. If you get more than one, there will be a cage match to the noisy death. Just sayin'.