A few thoughts....
Find a local machine shop or hobby machinist and have them increase the depth of the hole using a 2 flute end mill. They will also have the tooling to make sure you have enough meat there to go deep enough. Just bring in the trigger guard, talk to the owner (small shops are best) and offer to buy a couple of pizza's for lunch for the guys...
Another thought it look on the Brownells site you can probably find something in their screw selections that will be long enough and similar enough. Might have to buy a package of 8 or 10 but again not too expensive. I have even hit on screws I needed for guns at my local hardware store. Although not exact they have been close enough and the correct thread and pitch.
Finally.. Post the size, Threads per inch and length you need along with head style here. You might find someone has something in their junk box that will fit the bill and is willing to send it to you for the cost of postage, or less.

Just some thoughts from somone who has been there, done that!!!!

DonT