what kind of racing do you plan on doing? that will help answer your question better, because different sanctions require different spec cages.. the autopower might not be legal for what you are looking 2 do w/ it.
maybe you can find and contact the guys in the uk, with their rally sf. i dont believe there are that many sf's floating around caged up...
Making your own is a LOT of work. I don't recommend it. Autopower stuff is generally ok. One of my friends has used a couple of theirs with good fit results. I installed a Safety Devices brand in my old Audi and it fit like a glove. Kirk Racing also makes some good stuff and will modify as you request as they are made to order.
I've done an autopower roll bar in a GD WRX, and it wasn't tough. I'd assume it'd be similar for the forester.
What I'd recommend is getting one a bit more robust. I personally don't like the bolt-in style of the autopowers, and they also tend to have the bars come down in front of the dash which really hampers entry/exit quite a bit. If you don't want to hack up the dash that may be the only option though.