I built my custom enclosures out of cabinet grade plywood. I paid 25$ for a 4x8 sheet which is enough for 1 4x2x1.5 cage... In terms of a rack... check out sears... I paid 80$ for a 5 tier shelving system that works like a champ... For the work and time your gonna put into building one... Id personally just buy one... Mine is just like this just not as long... i believe mines 4ft long and this ones 5... With these you can adjust the shelving so it would work nicely with different sized tubs.... As far as heating... i think if you go with 11inch flexwatt u may need a bigger tub. I use 60qts and 11inch flexwatt covers just about 2/3 of the bottom.

http://www.sears.com/shc/s/p_10153_1...=storage+shelf