Rifle Rods! (https://www.amazon.ca/Gun-Storage-Solutions-GSRR10SK-Starter/dp/B00570T3R4) Don't buy off amazon though. Also you can make your own with dollar store balloon sticks and either velcro or, my preference, magnets, along with coat hanger to make the balloon stick stiffer.
I haven't used them myself, but there is these:
https://www.storemoreguns.com/the-rifle-rods-gun-rack-system/
https://www.amazon.com/Gun-Storage-Solutions-Starter-19-Inch/dp/B00570T3R4
Check out the middle picture on the amazon link (same product)
I like this, I have the velcro option, and I have to re staple it to the top very now and then. That, and my SBRs require a lot under them to keep them stable. I just try to leave the silencers on them and that helps.
Here is my Velcro setup. It was $40 on amazon, overpriced for the material involved, but came prime in a day so that was nice.
http://www.amazon.com/dp/B00570T3R4/ref=cm_sw_r_awd_2bVhub1PGR2SR
I used these rifle rods in my stack in locker. I glued them to a make sift shelf and has worked great so far.
Pretty easy set up, and you can fit twice as many guns in your safe. It also seems like they are easier and faster to get out of the safe.
http://www.amazon.com/Gun-Storage-Solutions-Starter-19-Inch/dp/B00570T3R4