I am attempting to come up with something similar but not sure of a weight limit.. was looking into adjustable clothing bar; then opted for a stronger version that is used to hold cargo in place.. will adapt the good rubber lined gun holders that fit atv racks or handle bars.. I will post if it works, with some pics.
under the steering column seems a good spot, not easily seen and really accessible when seated in the auto.. front of the seat would be a good spot for access also.. I usually hang my hunting pistol in its holster over the passenger seat and easily in reach, either in front of the seatback or behin d it depending on my needs..
behind my bucket seats I have two hanging cloth storage packs, with a closing pocket at the top and three loops below it for long guns to fit with the stock and barrel supported by the passenger and driver loops.. stable and cool, just not as easy to access with speed and or economy of movement.. I place a small pistol or binoculars/ ammo etc in the top pouch and two long guns behind the two seats.. works for me..