Very nice work. My own $00.02 worth of advice is to avoid using any urethane product. You will be surprised how quickly ordinary water will dissolve a plastic finish. A much better solution is traditional Tongue oil. I have used it on several bits of gear that are in my sink with water flowing over them at times, and the wood is good as new still.
All my darkroom surfaces are finished with several coats of marine spar varnish (first coat cut in half with thinner to help penetration). At 20+ years old, they still look like they did when new, except developer will stain if left to dry, but the surface is still good.
I second the fiberglass screens. I had some like this years ago (I hang my prints now), wooden frames. I just stacked them, but this will allow more air circulation. (I didn't finish the frames, never a problem.) You need to be able to clean the fiberglass with a weak clorox solution from time to time to eliminate contamination.
Nice. I am going to building the same thing next week, from the same book! Hope mine comes out as well as yours did ...
Building something is always good.
However, this is a better picture of something simiilar to what I found. It is called a "Pan Rack" I put window screens in it.
It takes 5 or 6 coats of Tung Oil before it has any moisture resistance. Each coat take a long time to dry. Its protection is via penetration. There is a reason they use varnish on boats. I would use an epoxy finish, seals at the surface and last nearly forever. I have had a sink that I epoxied 30 years ago that still look about the same as the day I did it. Gave it away last year, still going strong in a new darkroom. Just my 2 cents
Buy fiberglass screening.
Test out the drying technique before commiting tons of time to screen construction.