I see the issue and the reason for the wood on top although I really doubt the batteries will jump up. But here's a few thoughts.
You could stay with the wood on top.
Or use something like this. If the floor is just thin steel I would through bolt the metal loops.
LinkBuy a battery box and just set the lid in top of the batteries. This is basically the same as the wood mentioned above.
Using short screws from the top screw the wood to the bottom side of the shelf.
Glue a piece of cheap veneer to the bottom of the shelf above the batteries.
Duct tape the batteries down.
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2001 Suburban 4x4. 6.0L, 4.10 3/4 ton **** 2005 Jayco Jay Flight 27BH **** 1986 Coleman Columbia Popup