Basically:

Put the sand, snow or whatever in a ziplock bag.  Clip off one bottom corner and pinch shut with your fingers.  Now spread the snow across the display through the hole.  Just pinch off when you're done.  Clip with a plastic chip bag clip on the corner to close it.

If you use a large enough bag then you can more easily stop the flow, kind of like the squeeze tubes used for decorating cakes (which, by the way, wouldn't work as well because you can't squeeze off the end as easily to stop the flow).