Hey Pat. Yes, they are expensive in the scheme of things but well worth the $ in my opinion. The real ATG guns can use different sizes of tape if you get the "adaptor" that lets it use bigger. One roll of the tape can last quite a while as well. (Depending of course how much you are using it regularly of course
) They are basically a "double sided" tape that you "roll" out on your project. It is sticky as soon as it is down - no exposure to air needed.
They are the tape guns that a lot of framing places use (which is why you tend to find them online at framing supply stores.) Unfortunately, the real ATG tape guns themselves (that have the adaptor to fit and accept different sizes of tapes) are not usually sold somewhere where you can use a coupon. However, there is a pink version that some of the craft stores sell that you could use coupon for. I believe it only works with the smaller 1/4" tape. They also sell the refills of course at the store that you could also use the coupons on. These would be the tape guns that many of the forums used to do "share buys" for because if you bought in big enough bulk you paid less per roll.
It is a fairly strong adhesive and would work I think to adhere your template pieces together. It would be similar to the "red line tape" (but not as strong). The red line tape might be another option and maybe better. Might give you stronger adhesion, you could "cut" it to fit areas as needed.
I have never been a big user of any "wet glues" as I don't think they adhere well often and would be messy and you would need to worry about it bleeding out between your pieces when you were first adhering them together which is why I thought about the ATG tape gun.