I'd just use some sort of metal loops for a base, and make a strap out of 1" Nylon webbing and Velcro - but I've got yards of the stuff around anyhow, and a machine that doesn't (usually...) choke when sewing it. That's how I also made the straps that hold my sub tube in - and the connections are done with "locking" banana plugs, so I can remove it when I need the room. Works great - I just strap it to the cargo loops on the driver's side when it's there, and I can pull it if I need to...
I see no reason why you can't do something similar for pretty much anything. CAVEAT - if you're going to do this for something you're likely to need in any sort of hurry (like a fire extinguisher...) leave an inch to an inch and a half of the web "loose" as a pull tab, and use only an inch or two of Velcro to secure it. Make sure it fully overlaps (for security,) but keep it short (for rapid access...)
Why Velcro? Because the more you shake it, the tighter it holds. It's nice like that...
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