cut a pop can in half and clamp it to the tail pipe with a small hole about the size of a quarter in it
use a clamp though and not duct tape or it wont be on there for long...
From what my camaro ran like during the brief time I had restrictors duct taped over my exhaust it made it run good at low rpms. The other guy who tested them noticed his car warmed up a lot faster with them on. I think the main benefit would be just the improved warm up time so the hole just needs to be small enough to give a slight back pressure to help build up some heat.
I might test this out if it gets cold and stays that way around here. But tomorrow it is back into the 50's