The heat should have worked. How hot does the heat gun get? Mine is 400F when I close the vent on the back almost all the way, and put on the attachment that squeezes the air down to a thin flow.

You have to get it really hot. Just do it slowly. Heat for several seconds with the end of the gun about 1-2 inches from the plastic. Move the heat gun around it constantly. Don't let it sit in one spot too long. Heat it a bit, then twist to see if it's loose. If not, heat again immediately, and repeat. I can usually get it off in about 5mins using this technique...

The paint thinner would certainly make it easier to come off, so if you just don't think you can get it with the heat, then follow PSI's advice, and it should come off for sure.

Just remember, a whole new frame and boom is only about $15.00, and that's your worse-case scenario. Don't be too afraid of breaking it...