I would work out how many rubs and what size I was going to use and then plot it out from there. Also think about where you are going to put it and then you can design it around these limiting factors.
Not sure about the shelves but i would route out channels for the heat cables on the actual shelves themselves as if you put them on the back wall it will just raise the ambient temperature more than anything.
There was a thread on the darkside for a rack it was quite good ill try find it for you
0.1 CB10 Snow Cornsnake - Galaxy ~(Currently MIA) 1.0 CB11 Ultramel Anery Consnake -Orion 0.1 CB11 Lavender Cornsnake - Star 0.1 Leopard Gecko-Simo 1.1 Jack Russell-T.J and Harley (Minature) 1.1 Cats-Rascal and Dizzee 1.0 TB Horse-Mickey