Interior Cargo Box
 

Off the top of my head would be to use the cargo net? if not maybe a bungee cord from the back 2 anchor points to form some sort of seat belt for the box.

Thought about bungee cords.

I would think some Velcro type "hook" tape installed on the top edge of the box would adhere to the carpet seated backs.

Now that's a good one but would it hold when stuff was in it?

you could have velcro on the back and bottom of the box. it should hold anything within reason

I assume the box bottom would not be floating mid air. If it is flat against cargo floor / mat, it will hold weight of items and not move.

Tried the velcro on the back but not the bottom as I have the rubber mat in there but it looks like the Velco will work good enough as you can't put a ton of weight in it anyway so thanks for the ideas everyone.

Neat, much better than duct tape:p