I have a '93 YJ and got some inexpensive tie downs at Napa Auto parts. I am sure any parts store would have them. They hook into a hole in the battery tray and screw down to a 'crossbar' that goes across the top of the battery.
Nope, there's nothin' holding the battery down except for the cables! I drove the jeep for about a day before noticing that one of the terminals made a scatch on the underside of the hood. I'll go down the the local parts store and see what I can rig up.
No need to rig anything up. On the bottom of the battery tray there should be a slot on each side. Get a pair of "J" bolts and the metal "strap" that goes across the top of the battery. The bottom of the "J" goes into the slot and grabs the tray. The "strap" goes across the top of the battery with the two "J" bolt's going up through it and a couple of nuts or wing nuts to hold it down.
There is usually a "kit" that you can buy for a few bucks.