I recently got a Zumo XT, and it's kind of a beefy unit. I wanted a mount that would really grab the accessory rail without putting all the pressure in one single spot. The mount that came with the unit only had a single u-bolt style mount, and the u-bolt is way bigger than it needed to be for the half inch rail. More surface area to grab a hold of makes for a tighter grip with less pressure put on the rail (you could probably put a dent in the rail if you really tightened down a single u-bolt). So, I made my own base mount for the RAM armature that came with the Zumo.
I bought a couple half inch 1/4-20 u-bolts at the hardware store, along with some 1/4-20 acorn nuts. In order to increase the surface area for gripping, I made a couple bushings for the inside of the u-bolt out of some scraps of ABS plastic. This is what the u-bolts looked like:
The base for the mount was made out of a scrap piece of 5/32 aluminum. I drilled four holes for the u-bolts, and tapped a hole in the center of the aluminum plate for a 1/4-20 threaded RAM ball I bought on Amazon. I spread some JB Weld on the threads and screwed the RAM ball into the mount; it's now permanently attached.
Here's that part of the mount:
I made a bushing for the underside of the plate, again to add more gripping power to the mount. The bushing is made from a bunch of pieces of scrap ABS sheet plastic. I drilled a half inch hole in the pieces, glued them all together, then cut off a small section to form the bushing. I glued the bushing to the plate with JB Weld.
Here's what the bushing looks like:
Here's the finished mount:
....and on the accessory rail:
Here's the Zumo XT mount attached to the rail mount with the RAM armature:
I wanted to keep the Zumo power cable out of the way to minimize the chance of it getting snagged on something, so I made a little cable attachment plate to bolt to the rail mount. It was made from a scrap piece of 1.5mm aluminum, and it fits over the ends of one of the u-bolts. I made a little cable clamp out of a scrap piece of ABS plastic; this fits over the Zumo power cable and is secured to the plate with a couple of 6-32 screws that fit into a couple tapped holes in the 1.5mm plate. For the portion of the cable that's exposed before it goes under the cowling, I ran it through a section of clear vinyl tubing for a little extra protection. The power cable is routed down to a PC-8 under the seat.
Here's the mount with the cable attachment plate:
It required very little tightening of the acorn nuts to get a really strong grip on the rail.