I still like the simplicity of a tube, but not the expense that it it sounding like.
Anyone know how much corrugated plastic drainage pipe is? It's perfectly strong enough for the use, wouldn't be too hard to cap with plywood, and can't possibly be that expensive since it's not made to withstand any sort of pressure.
A guy that stores his 16 where I do recently showed up with exactly what you are talking about. It is really a large diameter black tube, but he managed to mount it on the trailer. He seemed to find matching end caps, and they look great.
I have a PVC sailholder on my trailer that the former owner had purchased. One end is stationary ( a cap epoxied shut), and the other end is a shove-on cap with a stainless hunk of round bar that goes thru matching holes in the cap and tube. The stainless roundbar has a stop on one end and a hole drilled in the protruding end which allows for a small padlock - it works great.