I've tried two approaches based on turn-by-turn instructions à la Dakar Rally roadbooks. One was to print the instructions as a long strip, folded in half and placed in a plastic bag or laminated, and worn on a lanyard. That was fine until above 10mph, at which point it blew around everywhere including into my face. Some attach it to their thigh, although that's more useful for recumbent riders than upright.
For my second attempt I constructed my own roadbook device (I used Lego, but most people use metal or plastic boxes, knitting needles and general bodgery, or buy a kit), and threaded my route instructions into it. I made it about 8cm wide to fit onto my handlebars but be big enough to read the route. I homebrewed the instructions but there are some good websites/software to make them.
A map holder can be improvised from an icecream tub lid and a bulldog clip, held onto the handlebars with rubber bands cut from inner tubes, or old toestraps for a belt-and-braces solution. Or you could buy one of the clear plastic A5 or A4 pockets that velcro onto things; my cheap Halfords bar bag came with one of those which unfolds to about 23x23cm. All that folding wasn't brilliant for the covers of my Landranger maps, though.