Small ones are called Cue sheet holders.. a Cue , directions,
reduced to just miles to turns and directions to turn.

A Chart cover , is for Navigation charts, and they need to stay dry..
so is of clear waterproof stuff..
on Tours, I've used those , with big panel maps ,
like Irish & UK Ordinance Survey Maps
just some elastic cords to put it around stuff ..

I had a dry bag for my film camera bag on my front rack to secure the map to..