Hi everyone - I am building a heat shield for my corner-installed stove. I would like to use cement board floating 1" from the wall (and of course 1" from the floor and 3" from the ceiling) and tile over the board. However, the drywall in my house is not a flat surface. Heck, there's not one straight or square surface in this old house at all. So my question is how would I install the cement board to correct for any wonky surfaces? I was thinking of using metal or plastic washers to build up length on the ceramic spacers in places that the wall dips in, and I see that they make high-temp washers, but they are expensive and I was wondering if regular metal washers would do the trick, or if that would be unsafe? Does this even make sense? Thanks in advance for your advice!