I've seen a few posts go by about picking the right mount out for a particular ceiling grid, so I though I'd share a solution I pieced together a while back for a particularly annoying ceiling. (For the curious, the tiles managed to overhang the grid on all four sides, so the grid was completely inaccessible to any mounting brackets.)
Grab yourself one of these for each AP you need to mount:
Assemble the hardware. Make sure to remove one of the knock-outs in the electrical box so you can run your network cable properly.
Grab yourself a sacrificial ceiling tile, and cut out a single gang electrical box sized hole in the center.
When you mount the bracket, it will go in something like this.
A word of caution: if you actually see this view, you've gone too far! This picture is just to demonstrate how it will hang on the grid. You have to install your cut up tile first, then mount the bracket assembly on top of the tile through an adjacent opening. Make sure you have your network cable run. What you should actually see will look more like this:
Now you can follow the standard electrical box mounting directions for your AP mounting bracket. This one shown below is for an AP-225.
This lets you mount everything above the grid, without worrying about grid widths or how far below the grid the tiles hang.
