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I thought as you did on the felt absorbing moisture but unless you pour water on top of the surface you won't get moisture absorption. I used the mdf because the plywood alone had a tendency to warp. But there again I didn't seal it with wax. I've been using the felt for 15 years without incident. I use the felt in my planes cabinet for my planes with the same result and as an added feature it protects the blades too so I don't have to retract the blade and then reset it when I want to use the plane a little further down the road. Both of our techniques have merit, well done.
To help protect against the elements, after waxing I made a cover using 1/2" MDF and covered the bottom with felt. I added two stops on either end to hold the cover in place. I use this cover as a worktop for glue ups sharpening my tools, whatever I need it for. I also use a similar cover for my bandsaw but I use 1/8" or 1/4" plywood with the felt glued on. I live close to Houston, Tx where humidity is fairly high. The second pic you can see part of the top of my saw. It has been over 2 years since I waxed it. Used to live in Juneau, Ak where it rains about 95% of the time. I used the same thing there with equally good results. Also save those silica bags from shipping boxes to put in with your small hand tools.