Years ago, some small supplier produced a tone board for the 208R. I had one, it worked perfectly, and was easy to install. I am sorry I don' t recall who made it, and I seriously doubt they would still be available, so not much help in answer to your question. The 208R, although now would be considered relatively large and heavy, was / is an excellent HT; I hope you can make good use of yours.
73, HNY, Kent - K4MK