Funny; my dad came up with a great resolution to this awhile back. I actually had the piece that tries to hold the carpet in place, but that ripped right out and left a hole in the carpet. That gave us a point of entry to make a small bracket that the front left seat bracket bolt goes through on one end, and put a bolt that we cut down to size on the other that sticks up through said hole. Place the mat so the bolt goes through the hole in the mat (well, the stock subaru ones do, you could drill yours out?) and tighten a nut down onto it. Works like a friggin charm, I haven't had to bother with it since.