I use a HG all the time for big and small areas. Ultracote is a low temp film and requires careful use of the HG. Hit the area with heat and pull the HG away; the Uc will continue to shrink. If you keep the HG on until the wrinkle is gone you will overshrink it. Also UC (and MC) need to be burnished on solid surfaces. I have successfully ironed sections of UC together with a 1/4" overlap to create a sheet of covering with a pattern and then tightened it with the HG with no problem. It DOES take practice and patience.

I still have to use an iron to draw out those little corner wrinkles.
Walt