Need ideas for BT speaker mounting pls (With quick release & security)

[Goliath]

I would like to use an existing BT speaker I have that sounds great, gets more than loud enough, and can be controlled remotely. I don't want to mount a fixed speaker system in the cart.

Anyone have a good idea on a mounting style that allows quick removal, AND the ability to lock it into place for anti-theft purposes?

Here's some pic's of what I'm working with, and where I'd like to mount it.
I was thinking a strip of 1/4" aluminum between the two front seat back rails for the main support, but have never improvised a locking quick release system.

Any ideas appreciated!
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