I don’t use risers because I want to use all available heat so I can actually use LESS heat.
My UTH and RHP are all on the same side of each cage. That way, the RHP on the bottom cage is “prewarming” the floor on the cage above it, so less heat is needed from the UTH. And vice, versa. The UTH is prewarming the ceiling of the cage beneath, so less heat is needed from the RHP.