I use a $15 heat gun I bought from Harbour Freight to modify kydex. Its hot air instead of an open flame, and has two settings: low and high (AKA Patient and Impatient). I had to do something similar to a Blade-Tech holster to accommodate my Marvel Unit 1 .22 conversion on a 1911. The bbl sticks out by 1/2" to accept a comp. Heat up the end of the holster until the kydex is flexible, then shove the gun into it and hold while the kydex cools. If you wear a glove you can do some additional molding by pressing / shaping the kydex with your hand.
The heat gun also works pretty good for breaking down red lock-tite.
Flyfishdave's gun looks like the kind you use to finish off creme brulee... (for the record, I like creme brulee!)