I had the route sheet rubber banded to my forearm for my first audax, maybe my second too. It worked. Got a bit sweaty, and the rubber bands pulled at my arm hairs, but it was surprisingly workable.
My problem with route sheets these days is more to do with actually seeing them. I'm short sighted and wear glasses for driving. If I were corrective lenses on an audax, I can read road signs but not the route sheet. No glasses and I can read the route sheet but not the road signs. Damned if I do, damned if I don't! I need to invest in bifocals from Optilabs or somewhere!