Going custom isn't a bad idea, if you've got metal-bending experience. I know there's an informative thread over on Slowtwitch about what metal to buy and what tools you'll need. It will definitely be cheaper than buying pre-made extensions, plus you can get any shape and angle you want!

Good luck

PS- My Trek, or what's left of it, is a 1500 (2007 model). I bought it for both road riding and tris, but since then I've decided to go strictly triathlons with it, so I've ditched almost all of the stock parts except for the frame and some of the drivetrain. I probably would have been better off just buying a P2K from the get go! (Oops, live and learn )