I use covers made from a magnetic sheet cut to fit (lightduty stuff from office supply). My goal was not temperature control, but to keep debris from falling into the duct during the greater part of the year when we don't use the furnace. They have worked fine but I think if we walked on them a lot it would cut them. Great if I am into a project and I am tracking stuff in.
2011 Sabre Silhouette 260BHS
2006 F-250 6.OL