I used 1/2" ply, 2 X 2 squares, painted flat black (lesson learned: I wish the paint could have been flatter). Could have put the ply on top of the tile, but I worried that as you tighten the bracket to secure the speaker, you would be compressing the fibreglass and creating ugliness.

I also had to buy additional cross tees, couldn't find black so I bought white and spray painted them, that worked well.
Results:

Those are just $150/pair Polk speakers, though.