The "plutonium implosion device" we built was the size of a Volkswagen Beetle, weighed five tons and looked like this:
Even if you leave off all the "bomb" packaging, there's still no way it's gonna fit in the back of a Volkswagen.
Even a "bus".
Look up the “Davey Crockett” recoilless round. 56 kg and about as twice as big as a basketball.