Unfortunately, the battery "tray" is not separate. It's part of the plastic inner wheel well. One of the dumber GM ideas. Unless someone put one in. There were two sets of tabs depending on the battery physical size. Not uncommon for them to be snapped off over the years.
If using the correct battery hold down bar, it should keep it in place for the most part. The proper battery hold down is GM p/n 22512299 with HD battery, 22506770 without HD battery. NOS ones are a bit pricey. But the repro isn't so bad.
EDIT: oh, I see you have a clamp in there. Can't tell which it is. The number is molded right into the top of it. The HD battery is a tad shorter. And hard to tell, but it appears yours is the 299 already. If your battery is slipping around, maybe a strip of velcro under the battery could help keep it from moving?
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