You can see here, where it doesn't fit the OEM battery tray:
The battery has a flange on both sides. It hooks right into the retainer on the fender side but is just a hair too wide to slip into the lip on the inboard side.
So. I will probably just trim this tray and put nut-plates under the holes you see in the battery flange and bolt it down with a thin strip of metal. Remember, it's already hooked under the lip on the fender side.