It's funny because there are some parts of the country were moss grows everywhere and people try to discourage it. Actually, I had to reread the word, because I was thinking "mold." I think that mold grows in just about everyone's bathroom. Maybe you'll need to take longer showers. Or, maybe you need a humidifier.
Ok, now the picture has loaded... um, I would think that you wouldn't need the special shaped tray. My first thought was a cookie sheet, one of those sheet cake ones (15x10?). My only other thought was an under the bed storage container, but it's walls would be too high.
I would suggest cross-posting in gardening, since the indoor gardeners might know just the thing. Also post in mindful home management or recycling. I would also research the moss that you plan to use to make sure that it doesn't have anything wierd that would effect your family or pets.
Otherwise, I think that it looks very interesting.