Oh woops, I totally misunderstood the OP. I thought he was trying to mount them in tubs in a rack lol. Just with tubs with a lid, pretty much what BPgator said. It would be hard. You would probably need to mount the rhp to a piece of acrylic, then mount the acrylic to the inside of the lid of the tub. Then on the outside of the lid you would probably have to mount 2 flat pieces of aluminum rod which would stretch perpendicular to the length of the tub so they put the weight of the rhp on the sides of the tub. Would be way more work than its worth. Now if you were using something like a small 25 watt RHP, you could probably mount that to a sheet of acrylic on the top of the tub. I saw guys who mounted them to the top of Exo Terra cubes using acrylic sheets in place of the screen top and it works out well.