I've rehabbed a lot of properties with plaster walls. What I would do here is cut this so it's somewhat of an even rectangle with a stud at each end. Remove the lath strips. Cut a sheet of drywall thick enough to come close to the finished wall without sticking out and hang it up. Depending on the wall/thickness, you may be able to keep the lath strips in there or use them to bump out the drywall to meet the wall.

Tape up the joints with fiberglass tape. There's a plaster/joint compound called "one pass wall repair" that does really well with plaster. Use very small batches as it sets up fast. Use that to blend/skimcoat the new drywall. You can get away with drywall mud, but the one pass blends better. Let it set, wait a day, sand it out, repeat until it looks good, paint.