To hold the mobile sub box in place in both cars, I suggest using industrial strength Velcro. Good prices for the 2" wide strips at
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Buy some 1" or 1 1/2" wide, 1/16" thick aluminum bar stock at your hardware store and cut it to whatever length you want to make two strips for the floor mounts. Pre-drill holes for sheet metal screws in each aluminum strip and countersink the holes, then cover the aluminum pieces with the soft (loop side) Velcro so that the edges wrap under the aluminum bars. The fully Velcro covered side goes up, the side with the Velcro edges wrapped around goes down. Use flat head sheet metal screws poked through the Velcro and through the holes in the aluminum strip under the Velcro, and fasten the screws to your floor.
The hook side Velcro strips go on the underside of your sub enclosures, and the whole assembly can be easily moved from car to car using the connectors suggested by Don.
If you doubt the strength and reliability of Velcro to securely hold the sub enclosures, this is the method that I used to hold two powered Bazooka subs to the tops of behind-the-wheelwells storage boxes in a Ford Bronco.
A strip of hurricane fence material hanging out in the roadway snagged my right front tire, blew it up, broke the aluminum wheel, and tore the tire, wheel, and right side axle completely off the truck and stopped it dead. The subs stayed right where they were.
The driver right behind me was a CHP officer who kept oncoming traffic from driving right into my wreck. First time I was ever glad to have a cop right on my tail.