There might be some situations where you want your rokushaku fundoshi to have a little more coverage. I posted about the langot a couple weeks ago, which is one option from India, but if you'd rather just adapt your existing fundoshi, there is a simple way:
After you've cinched the fundoshi at the small of your back and let the apron portion fall from your shoulder (that you would normally wind around the thong and belt to finish the fundoshi), keep it flat and pull it backwards between your legs. Pass it over the belt portion at your back, tuck it in and pull it flat. It'll be against the skin, between you and the thong portion that you previously tied.
This works great for lounging, though if you're more active the back portion will work its way into more of a wedge shape. Not entirely bad, really -- it's a good look that strikes a middle ground between rokushaku fundoshi, mokko fundoshi, and a langot. Looks great in white as well as orange!