Back in 2000 when we drove to AK I made inserts for the living room and bedroom vents out of heavy duty cardboard. They were cut to fit the opening and about two inches from one edge I sliced halfway through the cardboard to make a hinge and used tape to make a tab to pull the inserts out. DW didn't know I had and when she realized how simple insisted on one for the bath. Easy to find the material and make the insert while on the road. Still using them but, I like dbates idea of using the 1/2" construction foam even more.
