Well, there's always matte polyurethane or spar varnish. Both are very tough, basically waterproof & not too hard to work with.
I thin poly slightly with thinner or mineral spirits & wipe it on with a lint-free cloth. Approximately 3:1 or 4:1 poly/thinner. No brush marks & quick drying. It will require an extra coat or two for the same thickness as unthinned though. Afterwards buff with a fine Scothbrite pad or similar for an even, matte finish.
A small can is fairly cheap. You could experiment on a scrap of hardwood & see what you think.