Straight from Czechia - the best way to hold a map on your bike while navigating.

PilotOne SH 30 mapholder - map is 14 cm (5,5") above the bar axis and board size is 305x305 mm (12x12"). 30 board is bestseller in MTBO community, with one hand only you can easily manipulate with A3 size map. Attaches to 31.8mm bars. 

https://www.mapholders.cz/product-en/pilotone-mapholders/tb---tallboy/pilotone-tb30/22