Since I've seen a bunch of people asking, I did a little bit of research into how big the hole for the temp controller ought to be cut. Since it was near impossible to search this thread to find the answer, please excuse my bit of SEO at the bottom.
According to this page
, the controller's faceplate is 75mm(wide) x 34.5mm(tall), and the body of the temperature controller is 71mm(wide) x 29mm(tall) x 85mm(deep).
My best conversion of those measurements into inches is somewhat imprecise. The faceplate is just a bit bigger than 2 15/16"(wide) by 1 5/16"(tall). The body of the temperature controller is just a bit smaller than 2 13/16"(wide) by 1 3/16"(tall). Going between these numbers give us an optimal "blind" cut of 2 7/8"(wide) by 1 1/4"(tall). Ideally you'd trace the box, but in terms of making a cut that's safe and pretty much guaranteed to work (assuming the measurements on the site are correct) this size hole should work. So long as you're using the boxes discussed in this thread, the depth shouldn't be much of an issue, but it's roughly 3 3/8".
So ya, assuming the website is correct, that's the best bet you have to cut the hole before you have the temperature controller in hand.
As far as cutting the hole, my plan is to use my dremel along with this attachment
and an impromptu wood jig to cut the hole. I'll report back to let you all know how it goes.
SEO: The size of the temperature controller, the dimensions of the temperature controller, the hole for the temperature controller should be, cut the hole, the size of the hole... Anyone have other ideas?