Here's a couple of my rack systems. There are two main types of heat systems used in home-made racks: 1) Cable and 2) Tape.
The white rack system on the left uses a cable type heating element. The cable is threaded through the holes on the sides of the system. There is a groove cut into the floor of the setup. This groove accomodates the cable. The cable is plugged into a dimmer and the heat is adjusted via the dimmer.
The rack system on the right uses "tape" which is sold by reptile suppliers. There are different widths, although 2 and 4 inch sizes seem to be most popular. When using tape, a clamp system clasps each side of the tape. The clamps are wired and the wire ends in a plug, which is plugged into a dimmer. Like the other setup, a groove runs through the floor to accomodate the tape. Even though tape is very thin, it cannot have anything resting on it and applying pressure - otherwise it will melt.
The standard sizes for rack systems are 32 quart sweaterboxes and plastic shoeboxes for baby racks. Different manufacturers (Rubbermaid, Sterilite, etc.) make boxes, which can vary in dimensions. For this reason, the rack system needs to be built to accommodate the boxes (not the other way around - because you may never find boxes to fit your rack).
The rack on the right utlizes clear plastic tubs used for salad bars. The tubs are rather expensive (compared to plastic), but are clear, rather than transluscent, so they offer me a good view of the snakes. The floor size is 10 x 20 inches and these are ideal for yearlings and smaller types of snakes (hognose, red milks, etc.)
Many people have "adult" racks as well as "baby" racks, but if you have a decent-sized collection, it's useful to have an "in between" size rack system too.
Tim

Third Eye