I took a nice thick oak plank to a wood shop and had them router a 1.25" half circle shaped groove in it to match the tubing on which I mounted it. Then bolted it with Scotty rail mounts on the other side of the tube from the grooved plank. I touch up the varnish on the oak from time to time, but it's been holding my 22lb outboard perfectly for 17 years now.
Dream Catcher (Nordic Tug 37)
New Moon (Bounty 257, Volvo KAD44P)
"Cruising in a Big Way"