Thanks Pat, running into a snag.
Today I popped open the tent and hooked up the new box to the battery.
That extra alloy lid that came with the box worked out like I thought it would.
It wedged in between the mattress and holds the box up.
I plugged in the 12 tent light and it works in any of the three outlets.
But as soon as I plugged in the 12 v bunk pad warmer, BAM!, it blows the 30 amp fuse up by the battery.
This was with the tents light running too, and I would imagine the light can not be pulling over 1/2 amp?
The warmer pad pulls 7 amps max or so.
I am suspecting the pad is defective or the plug is.
I did not have time to investigate it further, but I will be ordering a 12v extension cord that goes right to the battery, and I will see what happens when I plug the bunk pad directly to it.
If it blows, I will cut off the bunk pad cigg plug and wire in a new one.
I do not think it can be my wiring of the box, as I mentioned all three sockets worked when I tested out the tents light.