If anyone cares, You can take a 16" ar15 with a collapsable stock and Mount in on the underside of an Add-A-Trunk in a wrangler.  I used Two heavy duty spring loaded mounts that are made to hang yard tools in your garage.  One for the barrel and one holds at the tube on the collabsable stock.  You can not see that it is there unless you damn near lay down on the ground and look up.  It has never fallen down.  

P.S.  A bit of Shrink wrap over the spring clips will keep them from scratching the finish

This could be dangerous to your barrel, I've been told never to let the barrel support the weight or partial weight of a weapon much less supporting it's weight in a vehicle that probably hits a fair share of bumps and rocks.