I have some magnetic gun holders. You secure them to the base with the included hardware and then your firearm sticks to it. Great for under a desk or in a boat or vehicle. The magnets are coated so they don't mess up your guns. I have a couple different models. They're strong and can hold a rifle easily. 

$10 per pack of 2 magnets (the Hill & Wood and Tac Pro models - the first and second magnets shown in the photo lineup)

$20 per package that has 4 magnets (the GunPro model)

I also have a wall mount AR15 holder that fits the magwell and lets the rifle sit securely - $10 each