Just get a piece of wood dowel from the hardware store. It doesn't have to be oak. Or a scrap handle from your things. Around 3/4" dia. or a little less.

Drilling into a dowel stock evenly is not easy. Cut a chunk and go half way in from each side with a small drill. Then a larger drill bit all the way through. Coat the outside with epoxy and fit it in. Once you epoxy that in, use a Q-tip and coat the inside passage.

You could also use the a piece of the grey plastic electricians pipe. If your distance is small, you could use the male end chunk which is a thicker wall. I saw a boat with that and it looked fine. After all, we do use plastic deck plates and synthetic bungee. Soften the inner edge if you use that.

Seats and backbands etc. Thats a trial and error thing that you work through and finally find a nice solution that feels good to your anatomy. Even the distance between the back band to the seat just takes experimentation.