I use a sawsall with a fine metal cutting blade. The finer the blade, the finer the cut.
Tape or clamp a straight edge like a piece of wood as a guide to get a perfect cut.
If you use a surface grinder (not recomended, and you shouldn't, but people use them anyways) with a cutting wheel, use both a face shield AND safety glasses. Chunks fly and you'll get hit with molten hot aluminum.
The sawsall works best & cut the backside of the Diamond plate.
I won't be out your way until Mid June,, but you can use one of mine if it helps any.
You can buy them reasonably at Home Depot.