So Dean down at Evert’s was kind enough to loan me a Folbe so I could figure out how to mount them on my pontoon. I don’t have a gunwale, I have aluminum tubing. And since the Folbe rail/tube mount kit is $10/holder extra (which would total up to $80 by the time I’m done), I figured I’d see what I could do with u-bolts.
When I got to the hardware store I looked around and found the size I needed to fit the Folbe mounting holes (1-1/8″ inch spread, 1/4″ bolts, 2-1/4″ from belly to tip – 3rd picture above), but there was an issue. My rail is 1″ tubing and the u-bolt is 1-1/8, so there’ll be slop. What’s more the backside of the u-bolt is curved and my rail is square. I don’t want dimples on my railings.
I could have used square-top u-bolts, but they were out of stock and still likely to damage the railings on my boat. Granted she ain’t no looker, but I don’t want to make anything worse than it already is. 
So what I ended up doing was getting some clear vinyl tubing with 1/16″ wall thickness, one piece of 1/4″ID for the bolts and one piece with 1/2″ID to cover the plate. Then, instead of putting the plate on the threaded side of the u-bolt like normal, I put it on before so it created a backing plate, keeping the curve in the u-bolt from dimpling my rail. Putting a 1″ piece of the bigger vinyl tubing over it gave it some traction (better than metal-on-metal) and further protected the aluminum.
Next I put a 1-1/6″ long piece of the smaller tubing around the legs on the u-bolt. Since my rails are 1″ and the piece of tubing I used was 1-1/16″, when I tightened the nuts down against the Folbe side mount, it squished the tubing lengthwise against the plate at the bottom and, with no place else to go, the tubing just compressed and the walls thickened to further cement the holder in place.
The only thing left to do then was to push the u-bolts onto the rail, wiggle them thru the Folbe side-mount, tighten down the nylock nuts I got so I wouldn’t have to use lock washers, and boom.
So for $3.91 instead of $10, the thing is so solid the railing would come off the boat before the mount would come off the railing, but it will never loosen, rattle, scrape, or damage the railings on the boat. For $20 I can do 5 mounts like this, instead of the $50 it would cost using Folbe’s tube-mount kit.
I’m pleased with it, it works and it looks like it belongs… at least on my 30-year-old, very utilitarian boat. 
I have to say we LOVE our FOLBE holders, much easier access than other holders.
