Oh, I see your point, (yeah that minimum order of $100 made me choke on my beer).
The size of the heat mat you want should cover 1/3 of the length of the tank, and no more. A little less that that is okay too, just as long as there is enough area for the WHOLE snake to get on top of all at once. This leaves a warm area right above the heat pad, a warmish area next to, and a cool area farthest, creating that temp gradient everyone talks about.
Since you would need a heat pad about 10 inches long, what I would do is get the bigger one and stick it to a plate of glass slightly bigger than the size of the heatmat instead of directly onto your custom made terrarium. This would allow you to freely move around the heat pad (it'll be just like buying the cobra mat, but it'll be heavier and made of glass). You can slide just 10 inches under the terrarium and leave the rest outside and that should be just fine (just remember to elevate things so the wire doesn't get crushed and cause a fire). ZooMed does make okay products and they seem to be a very popular company here in the US as well, so I wouldn't worry about it burning out on you, plus I think they come with a warranty in case it does.
Hope that helps, I'm pretty sure if you think on it long enough you might be able to find a better method, but that method I used since I was little until I found the cobra heat mats and it's worked pretty well.
-13mur 6