i've played with this idea off and on for brevets...
after a few 3d models and sketches never came up with a workable, easy solution...
went back to an aloksak and a home brew cue sheet holder bent out of a fender strut:
i've always wanted to try
this one, or make a smaller one:
but trick is balancing weight / cost / bulk / waterproofness / complexity to use, change when tired / cold / etc. - as well as being able to add notes or minor changes to the route...