Only a thought or two. If it's not a particularly valuable map and you want to mount it on a sheet of ply, maybe you could trim the map to four feet wide? Maybe, use white faced MDF so it doesn't telegraph color or grain. I'd suspect any glue or other curing finish would make paper a tad translucent, and quality smooth ply or five foot wide ply might get pricey just to get covered up. I know sheet products can be edge joined, but it'll take fiddling not to telegraph the joint.
For tidying it up, someone could neatly tack some plainish 1/4" trim around the edge to 'frame' it, just as if someone might want to cover the layers of ply that would show on the edge. If there's some map legend or other desireable info that gets trimmed off. It might be able to be repositioned on some featureless part of the map off of the river. I've saved a smaller map to do similar at some point only for sentimental reasons, and it wouldn't bother me if someone has to chuck it in a dumpster down the road. Best of luck with it.
