Well, when acoustic guitars are built these days, the guitar maker looks for the straightest grained wood he can find. I imagine Renaissance lute makers had the same goals. As to modeling the strings, I suppose a long five-sided tube, set smooth, would serve.
good point orinoco and the veins of the wood would be running in the same direction as the instrument.
about the strings simple cilinders would do the trick.
Why on earth do you think they wouldn’t use varnish? Varnish protects the wood, keeps it from splitting, drying out, shrinking and expanding… besides making it shiny. Varnish recipies have been around for thousands of years (not hundreds – thousands). Wood instrument makers would surely have used varnish in the Renaissance.