>>I just finished my rack but I made it to tight so I am afraid to put it underneath is it ok to put it on the back on on the side of the rubbermaids.
This will probably be fine. I recommend attaching the flexwatt to the rack first, covering it and the entire back of the rack with a piece of expanded PVC (aka Sintra from a sign shop) and then placing a reflective-type insulation over that. Either Reflectix foil/bubble insulation or the foil-covered, styrene insulation board you can get at any HIC.
The Sintra will absorb and distribute the heat, the reflective insulation will direct it towards the back of the boxes.
>>My second question is if it is to tight can you put anything on the plastic to make it move in and out faster.
Unfortunately I have some experience with this. I once made the sides of a rack from some 1x4's and did not let them acclimate to the heat in my reptile room first. As such they shrunk and many of the levels became much too tight for my boxes. Rookie mistake.
I suppose a silicone or Teflon based spray might be worth trying, I don't think it will hurt anything.
What I did was a lot of experimenting and found which boxes fit in which levels the best. There are differences, believe it or not. Then I labeled them to keep them organized.
I also sanded and buffed the bottom and top rim of each box. This worked surprisingly well.
But eventually I took the racks apart and reassembled them once I was confident the wood could not shrink anymore. I'm glad to say they're still holding snakes almost 12 years later.
And I now build racks with large gaps. Always easier to fill too large a gap then to remove material, IMO.
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Current snakes:
0.2 Gonyosoma oxycephala - (Silver/Yellow)
3.3 Gonyosoma oxycephala - (Green)
2.1 Gonyosoma janseni - (Black)
3.3 Gonyosoma janseni - (Black & Tan)