I don't know if anyone has posted anything like this, and I wouldn't know how to search for it. But I thought I'd share my trunk gun setup with you all. Tacoma 2nd Gen Double Cab.
All it takes is one easy hole drilled in the plastic to hold the muzzle. The buffer tube in front of the stock and the FSB I tied to the body hooks with velcro straps. Plenty of clearance for optic, wider stock, badger latch, etc. This one is a 14.5" barrel with a welded muzzle device to go 16.1" but I haven't tried it with a 16" barrel. Does not rattle or move around and there's plenty of room left for other things.