I like that giant rubbermaid bin idea. If only they had a strap to carry on your shoulder, and/or internal internal organization dividers. If they were thicker then they would make good seats.
But to answer the OP's question, all dedicated gun bags and cases have something to stop the guns from moving around. It is more than likely foam or straps. It is a better idea to keep rifles in an actual gun bags/cases. Although I have seen guns in guitar cases, bat bags, tennis racket bags, suitcases, etc... Pistols should get their own bags or cases. Just to be safe, you should remove the mags when transporting guns.