light near a tank I would think. worst comes to worst, could you not keep it in a ball somewhere in the corner of the tank? I've done that before too. I stuck mine in the corner of the tank, but the UGF stack and where I keep my heater. I think I still have some there.
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.