There are two things I can think of right off the bat. I assume you are talking about a 8" circular hole.
1) Measure out an inch or two on either side of the hole, draw out a perfect square, secure the outside edges of the square with plaster washers or drywall screws so that the plaster is secure against the lath. Then cut away the square with a razor knife and straight edge. Patch the area with a square of drywall making sure to cut a hole for your workbox. Tape and mud as usual.
2) Try to find metal lath. It looks like screening but the holes are about 1/2" diamonds. THe lathe is made to secure new plaster. Apply a base coat, and then a top coat of white plaster.
I think #1 is the easiest to do. I recently repaired a 4ft x 4ft patch in my ceiling. If you get the hole perfectly square, you are home free.