Perchy has it I think. I’d go to one of those Nickel and Dime type stores and buy the biggest/cheapest framed poster and take out the plastic ‘glass stuff’ it’s framed behind and use that. Although to be fair my missus just bought a 120cm by 120cm polycarbonate sheet for £30 From Easter Rd plastics.
Alternatively you could but a waterproof version of the map and stick it straight on.
Or do what i said first but make it removable so you can put a map in local to the place you’re visiting.
Varnish will inevitably go yellow. Anything epoxy will also yellow over time, chemistry innit.