Hi Matt,
I think I can help you out there. I had the same problem wanting to do the same thing - heat a number of enclosures with heatcord - you are right, it gets a bit much using multiple heatmats. I asked a few related questions regarding lengths/wattage/amount of the heatcord needed and got varied, and not terribly accurate answers. Therefore I've worked it out on my own.
Hopefully what I have done will help you work out what you need (this is the info I would have liked myself!):
I have slotted shelving 1300 long with particleboard shelves, one with 4x 30x30x45 Exo Terras side by side, the other with 4 smaller plastic tubs.
The Exo shelf is using an 80w heatcord, routed into 5 slots, approx 2cm apart (obviously 1300mm long) in particle board (the rest of the cord is routed in a single slot on the shelf above to give some minimal artificial heat to frog tanks, but more because I had the excess length I had to do something with!). This heats about 1/3 of the rear of the tanks. This gives me 28-30deg in the hot spot, down to mid 20's in the cool.
The shelf with the tubs is using nearly 3x 1300 shelf lengths (routed again into the particleboard) of only a 25w heatcord (Due to the length of the cord not all being heated the last tub is getting 2 and a bit lengths under it). Once again, these are sitting around the same temps on the substrate (a few cm of peat/sand) as the exos.
I was quite surprised and it took a few changes of heatcord to do this as I underestimated the heat required for the exos - this is because the exos are so well ventilated (and glass) they loose a lot more heat than the tubs that only have a few dozen small holes drilled in the lids. I started off using shorter/cooler heatcord for the exos and it eventually got to the right temps, but only mid-day during warmish weather so really wasn't good enough, and clearly when day temps dropped would have been useless. I will point out I have not tested this setup in the middle of winter, but on colder nights the temps when I wake up have been perfect in the hot spot. I can't see it being a problem as you would expect to give them slightly cooler temps during winter anyway.
Since they are going to be on a thermostat anyway, just make sure you err on the side of getting a warmer cord than required.
Oh, and don't listen to anyone that says all heatcords are the same temp, just longer - its rubbish. To prove this, plug in a 4m 15w without a thermostat, as well as a 6m 50w, give them time to get to full temp, then hold each in your hand - they do NOT give out the same temp per area of cord - the 15w is hardly warm, the 50w will be very hot in your hand. I think people get confused by feeling the warmth when they are plugged into a thermostat - but any cord will only get to the thermostats set temps!
Hope this helps. PM me if you have any more questions.
