I'm replacing an exterior light fixture with an outlet to use to power string lights on a patio. The issue I'm facing is the exterior junction box I obtained can't screw onto the conduit as it's mostly recessed into the wall, and there's a lip that interferes with the rest of it. I was planning on attaching the box to a mounting block to even out the lip (PVC trim or whatever I can get that matches the flashing), but I don't know how to 'extend' the threaded connection so it can connect to the junction box. Is there some magic part I should look for in the store, or should I get a different type of box to mount?
Other code stuff if concerned: I've swapped out the standard breaker for a GFCI one (can't use a GFCI outlet 8' in the air).