4" diamond blade on a grinder. Get as deep as you can with that to your out line. Knock out the middle portion to expose the lath and snip that out with tin snips.
From what I understand, Roto Zip makes a smaller diamond wheel for thier grinder accessory. That might make it easier to get deeper on the shorter dimentions of the top and bottom of the box outline.
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Also Roto Zip makes a carbide router bit that will make it through the plaster well, but will get hung up on the lath.
[This message has been edited by e57 (edited 01-22-2006).]
