i see a lot of guys foil their lids and a lot of other guys giving them shit for it.
i used a few different things to see if any work.
aluminum foil works great to block the light from entering your tubs/buckets but it doesn't block heat as well and reflects poorly. some even claim that al foil's can concentrate reflected light and cause burns.
flex-fix class 1 duct tape is just that, duct tape. it's the stuff on the purple erkle. don't waste your time with this stuff. great for ducts but doesn't block light or heat at all and reflects poorly at best. it was lying there so i tried it...
metallic tape works best. i remembered i had some from a recent mopar build. its the stuff i use for firewall insulation:
http://summitracing.com/parts/SUM-350104/