There was a TBNer who rigged a rifle scabbard to one of the loader's masts (the vertical part of the frame that mounts on the tractor). It looked good, IIRC, and seemed to be pretty secure (unless he rolled the tractor). It also appeared to allow fast access to the rifle.
It's been quite a while and I don't recall how the scabbard was secured to the loader mast.
TC's suggestion about a 9mm is valid, depending on the type of holster. I carry a .38 in my pocket, but that's not easy access when I'm using the seat belt (which is almost all the time). If I were to carry a larger handgun (.357 Magnum revolver), I'd get one of those across the chest holsters some hunters use.