I use a hardware store milwaulkee heat gun, came with several attachments. I use the fan attachment turned vertical, temp setting high which is 1500 watts. It never gets completely smooth like a professional heat tunnel, but I figure its a small batch artisan sauce. The shrink band doesn't need to be exactly perfect. Like hand applied labels aren't all exactly placed the same or hand made soaps aren't all cut exactly the same. Obsess over making a perfect sauce every time and don't worry about a wrinkle in the shrink band.
Now, having said that...there may be some better or worse shrink bands to use. Different brands can behave differently. I'd be happy to send you some of what I use, a few in an envelope. Or, if you are interested, I'd love to trade a couple hundred for a bottle of your sauce.
Good luck!
SL