I made up some spacers to keep the batteries from sliding around, and strapped them in.
It works pretty well, but I may end up with some sort of turnbuckle hold-down to replace the straps.
The only problem occurs when going up a hill. The stock latch is completely inadequate to keep the tray from sliding open. I cannot understand how the manufacturer thinks that dinky latch can hold back 250lbs. You may remember that latch from when I installed the slide:
So, I added a couple of additional latches.
Made 'em out of 1" wide, 1/4" aluminum stock. Two pieces are VHB'd together to make a 1/2" thick unit. Perfect clearance. The pieces are long enough to just touch the top of the tray (under the lip) and the bottom of the base. They are held in place by 1/4" bolts and nylocks.