While you have the registers off check the installation job. The transition boot that comes through the floor most likely fits the hole "somewhat". Mine had big gaps where the extremely thin aluminum tape had torn away. I used duct tape that was intended for flexible fiberglass ducts, from Home Depot. Two or three layers of that sealed the openings well. Residential covers that close work for me. I like to close mine off in the summer because the cool air from the A/C falls in there and escapes to the underbelly, doing me no good. A scrap of styrofoam cut to fit tightly in the register cover seals it off and catches debris that can be easily vacuumed out.
2001 Ford F-350 Super Duty CC Dually, 7.3 Diesel
2013 832IKBS Flagstaff Classic Ultra Lite, Hensley Arrow Hitch, Carlisle tires
2016 days camped....211, it ain't over yet!
On the road to ALASKA May 6, home Sept. 13, 2016. 23 states, 3 Canadian Provinces, up to the Arctic Circle, 17,300 miles, 131 days on the road. 1 flat tire, 4 oil changes. What a beautiful trip!