There's no rule that say's you have to build everything on the boat. I have 5 kayaks and use the PD backbands on all of them. the cost is about $32. US. They're advertised in all paddle magazines and many paddling shops carry them. I find them very comfortable and easy to install and avoids one last headache I don't want to deal with.

Looking at your picture. (nice craftsmanship) When you learn to roll, you may not want a backband that extends higher than your coaming lip on the back. If you learn a lay back type of roll, the stiff and high backband will make rolling extremely difficult.