My girlfriend had rocker panels replaced on her Toyota Matrix , up here in Maine for $800. The guy did an outstanding job! I believe he used a "brake" to bend sheet steel to fit then welded. Toyota had plastic fairings that hid the rot. After painting, spray truck bed liner, then Fluid film the inside heavily. Then replace the plastic cladding.

 

Check out the Fitzee's Fabrications Youtube channel. He does a lot of rust repair on older style vehicles, but these techniques should transfer if you are looking to DIY on newer vehicles.

 

Some more pics of the extent of the rust

 

I’m right outside of Boston (Revere) and I had the same issues with an 09 Obxt that I had bought a couple years ago. Except for some odd reason the insides of the rocker panels were packed with wet soil (this car was a mess when I first got it). But after replacing the engine and getting a lot more work done on it I figured I would fix the body as well. I’m going up and over into the quarter panel once it gets warmer out. Here are a couple of pics of my progress on what’s done so far. Probably will go work onto the other side tomorrow.

 

That's exactly what I'm looking to do! And yeesh that pic with the jack stand is sketchy. Glad to see the metal in the area with the pinch weld is relatively flat and not too contoured, should make it easier to fab. What type of metal are you using and with what kind of welding?
At 200k my engine is getting pretty tired, specifically I believe the oil control rings are gummed up causing it to burn quite a bit of oil. Where did you get your replacement engine? I called a subaru specific shop in the area and he said their source for rebuilt engines doesn't sell to consumers.

 

Resurrecting this thread to share that I've bought a welder, and gotten some practice with it making this silly triple out of some bikes I found by the side of the road.

After build this and a few other little projects I feel like I've gained the confidence/skills/hubris I need to tackle the rear quarter rust. I took the wheel/liner/valence off to have a good look at the extent of the rust and make a game plan. I shot a couple videos going over the rust and my plan: