Before I bought my Prinsu, I put together a parts list to make one out of 80/20 parts. I ended up buying mine on a group buy for only a few dollars more.
My original design was almost exactly like a Prinsu but with more reinforcing. 80/20 has some corner pieces I was going to use as gussets (placed under the cross bars). My concern was the flex if I had to place something heavy, bowing the cross pieces and possibly permanently bending from dirt road driving. Next I was going to tie the crossbars with two "beams" running front to back. They would be cut to fit in between the cross bars.
My side rails were going to be 2x4 with the cross bars 1x3. The risers were going to be 3/8 aluminum plate, cut into 4 sections on each side, sitting in the roof gutter, clamped similar to the Prinsu with the side rails bolted inside of the risers.
Im guessing at the cost of building but I think with the 80/20, the aluminum plate, stainless hardware, powder coating some parts it was around 600-700. I really like my Prinsu but' I'm going to tie the cross bars down the center. I have already changed all the hardware over to stainless, run a rubber molding along the bottom of the riser where it sits on the drip rail and a heavy ground cable to the rack (I have 2 antenna mounted to it).
Obviously, mine was more expensive due to the size. The only obstacle will be mounting. I'm not familiar with the Subaru roof. Personally, I prefer building instead of buying and 80/20 is a grown up Lego set (or Erector Set if your old enough to remember). I believe this is very doable.
Scott