I've found that it's easier to build up the bit of your snorkus (the white bit the trumpet sticks onto) rather than trimming the trumpet. If you get some decent window seal foam you can wrap that around your snorkus and then, with good quality duct tape, tape the trumpet in. I've had mine for 10,000 miles with no issues at all.
... well, actually, I take that back. The only issue was that my first bit of foam i put on, I eventually found stuck to the bottom side of my air filter. MAKE SURE YOU ALWAYS HAVE A FILTER IN THERE! After my first try, I reapplied the foam much more securely and have not had an issue since.
Another option, which some may not recommend, is taking the WRX intake elbow ($80?) which removes the snorkus, and attaching that to your airbox. Then your trumpet would work with as few restrictions as possible. Then, however, you may come across some of the issues found when removing your snorkus.
Edit: My bad. You've got an '04 and no reason to worry about MAF errors. Carry on.
I believe he is talking about the rubber surround around the lip of the hood itself. i trimmed mine so more air could get there. So yes, trim the part where the trumpet is away.
I believe he is talking about the rubber surround around the lip of the hood itself. i trimmed mine so more air could get there. So yes, trim the part where the trumpet is away.