New medicine cabinet--old plaster and sheetrock help!
Drywall comes in different thicknesses from 1/4" to 5/8". You could use a couple of layers of whichever works out to the thickness you need......that would require buying a couple of sheets though. You can build up with what you have a setting type compound (powder) available at the big box stores. Use mesh tape over the joints between the old and new. A couple of coats of 20 (or 45) min. compound, skim with regular ready mix, and sand.
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