I'm not sure what specific type of mount you're using, but RAM makes a big selection of ball mounts to accommodate all sorts of applications, including one that will work with different diameter rails. Here's one:
Over the years, I've made a lot of my own mounts for various GPS units and phones. This is one I made for a Zumo XT that was constructed from a scrap piece of 5/32" aluminum plate and a couple of u-bolts from the hardware store, along with a 1/4-20 threaded ball mount from Amazon:
In order to make the mount grip the rail tighter, I made a bushing out of some scrap ABS plastic:
You could just as easily take a u-bolt and dip it in Plasti-Dip a few times to build up a thick coat on the non-threaded part and it would achieve the same result.
The trick is to increase the amount of friction between the rail and whatever clamps to it; the more friction you have, the less you have to really clamp down hard on the accessory rail. If you decide to use the split aluminum tubing idea, I recommend that you put a thin coat of something like E6000 adhesive on the inside surface of the aluminum tubing and let it dry before you install it on the rail. It'll increase the friction between the tubing and the rail, and prevent the tubing from slipping.
