I'd build a holster on the back of the bartop with enough of the gun sticking up to see that it's there and grab it easily.
Depending on where it is sitting, you could just drop the cord behind the bartop or build a box for it to drop into.
With the length of cord needed, I don't think any auto-winding cords are going to work.
Depending on how the top of the bartop is shaped and how deep it is, you could also just put some pegs on top of the cab to hold the gun in place.
(with it lying flat on top the cab).