I also have the HTC. I considered pulling the battery, but would prefer a quick fix, like a shielded enclosure.
Starting soon, you’ll need to show ID to buy tinfoil...
Well, I can tell you the antenna gets impeded quite easily with your hand. Just get metal that can ground both sides of the phone’s case together and that might work. The carrier antenna is housed in the plastic strip on the lower back of the unit, and the wifi antenna in the top half strip running above the camera.