There are a few options for painting open structures. My guess, the lightest of them would be Ultracote and then paint. Many people have done this with good results. I have done some striping in paint on Ultracote and Monocote in the past and painted stripes on the Ultracote on a sheeted wing on a Speeda Pattern plane many years ago. It works pretty well, though I don't know how well it would hold up in the long run.
Option 2 is a fabric covering. My favorite is Solartex. It goes on super easy and is tough as nails. We have also used it on a fuse where most of it was sheeted and the bottom is open stringers. We glassed the sheeted areas and then used the Solartex over the open areas and filled the seam where they met, and then painted everything. It held up very well. The fabric is heavier in my opinion, but it holds up great.
I have also used other fabrics like Koverall, Colortex and Worldtex. Other people like Stits fabric, but I've have never used it.
There may be other options, but this is what i can come up with off the top of my head.
