RHP are designed to generate verticle heat gradients not horizontal ones. It is quite likely that the 40w isn't enough power and also likely that if you went with a higher wattage you world have a heat unit inside the enclosure that was well over 100ºF. I suspect that the room temp is quite low and that is adding to the issues you are having. Is there any way to re-configure the system you are using? Maybe using the RHP as an ambient heater and adding a UTH as a hot spot heating unit might work better.