Update 24Oct2018: The suction cup holders have been field tested with photos attached. I'll remove the work in process status. If using the suction cups to which I linked below, their "sticking" ability was unreliable in colder weather and/or dusty windows. We found that removing the plastic suction cup "backer thing" did the trick for those that wouldn't stick, even in times of driving over rough terrain.

Know your laws, practice safe firearm handling, at minimum, keep firearms unloaded when not in use, actions open, safeties on. Use these at your own risk, I am not liable.

If you've ever been out hunting and decided to meet your party at the rendezvous point for a quick break, finding a place to conveniently store your unloaded weapon on safety for 5-30 min is not always easy. Leaning it on the side of a pickup or tailgate will often be the choice for most. If you've ever witnessed this, you've likely witness scratched automobile paint or worse. These holders are to prevent that from happening. There are 3 designs: 1. Suction cup design #1 to accommodate the plastic attachment which comes with the suction cup assembly (requires 3 nuts & bolts: https://www.mcmaster.com/90480a195 & suction cup assembly shown below) 2. Suction cup design #2 to eliminate the plastic attachment which comes with the suction cup assembly (requires 1 nut: https://www.mcmaster.com/90591a151 & suction cup assembly shown below) 3. Magnetic design #1 using 4 magnets (2 in each inset) instead of suction cups. I found it strongest to double-up on the magnets.

Link to suction cup assembly: Amazon Suction Cups.
Link to magnets: Amazon Magnets.
Link to Barrel Cord Locks: Amazon Barrel Cord Locks.
7/32 diameter shock cord (or equivalent)

To mount the suction cup holder variants, unscrew the center suction cup bolt until 1/4 remains in the nut (either embedded or via the adapter). Press and hold against surface. Tighten the holder while maintaining pressure until it becomes secured. Check for adhesion prior to every use, tighten as needed. The durable resin will warp if the shock cord remains tight overnight, use the barrel cord stops to release tension when not in use. These have not been field tested. As such, this thing is tagged as a work in progress. I will post any updates after testing them. Feel free to leave feedback in the comments if you test before me.

Found 32 3D-models for “accessory bar” across many websites

Warhammer 40K Barrel 1:55

Personalised accessory Carcassonne pieces

Portable Gun Barrel Holder

1/35 scale oil barrel

Under-bed Tray - Nozzle and Barrel Organizator Holder

Barrel

Single Barrel Stand

Plastic road barrier (scale 1/35)

Concrete road barrier (scale 1/35)

Mini Flicker LED Lantern on Barrel, Crate, etc

Barrels

Kral NP-03 picatinny rail