Tub Install Question

I have to pull out a metal/porcelain tub to do a tub surround job. Right now the tub is not recessed into the wall. I'd like to cut out the drywall to recess it, but then there would be approx a 1" gap between the top of the tub and the bottom of the drywall (1/2" thick) where the tub surround will not have any backer board. Is there some kind of thick waterproof material I can fasten or glue to the lip of the tub to bridge this gap? I guess putty tape put on double-thick might give the support I'm looking for. Of course, the final 1/8" inch will be caulked.

So I need something 3/4" wide and 1/2" thick to go all around the lip of the tub.

cement board glued on with the Loctite brand power grab adhesive found a Low*s