I'm replacing a shower stall in an alcove with a plastic modular unit. The old unit ended a few inches inside the stall, while the new unit extends right to the edge of the alcove. On one corner, the shower surround flange is nearly touching (the back of) the drywall on the other wall. The surround flange itself is about 1 3/8". There's also about 2" high of exposed stud at the bottom where the shower pan is.
How should I fill in the space on the surround at this corner? I was considering either trying to fit in a small strip of drywall, or filling it in with setting-type compound. I'd then put plastic corner bead on top. I might need to pre-drill some holes in the surround flange in order to attach the bead.