I have a couple different mosses in a 1 quart tupperware sitting on a windowsill. They are doing fine.
If you are worried about nutrients, put 1/2" to 1" of yard soil in the jar, swirl around, let it settle for a couple minutes, pour off the cloudy water/clay mix, refill with fresh water, and your good to go. (You don't have to pour off the clay/cloudy water, only if you are impatient to have all the soil settle and have clear water.) No soil cap/gravel needed if you don't plan on messing with this often and stirring things up.
-Dave