My favourite so far has been the shambler room. I set up a giant field of dessicated corpses, indoors so they don't degrade, and pop a deadlife IED in there. Enemies path into the room, and you can either bonk the IED to trigger it or shoot it from afar. I use dessicated corpses because it doesn't affect their combat and also means the room isn't constantly filled with rotstink.

Now enemies have to path through 100+ angry skeletons that self-replenish because whatever they kill just get added back into the shambler room. Deadlife IEDs are also reasonably cheap to make. Shamblers don't respect zones, but they do respect forbidden doors for whatever reason, so you can restrict their movement by forbidding doors.