The hold down in 700s is a long rod with a clamp fitting on the bottom. The clamp fitting just locks to the battery front lip on the bottom. The rear of the tray should have a little lip that holds the battery in the back. It's pretty basic.

The problem is, sometimes the battery trays corrode beyond recognition, and the hold down won't hold down (tray is rotten). You could try a universal hold down strap. In fact, you could just use a center strap around what's left of the battery tray to see if you could just hold the thing in place.
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Chris Herbst, in Wisconsin.