You take canvas...in your case maybe 40' x whatever width the canvas is.  So you have a 40' long rectangle.  Take two small strips of wood/pipe and run them lengthwise (40') and place an attachment point on each end of the wood.  Cut slots on one side of the canvas blanket or bag.  Sling the blanket/bag between two closely hung battons.  Now the 'snow' goes in the bottom of the sling formed.  When it's time to 'snow', you lower the batton on the slotted side and a small amount of the snow falls out. 

 

In the picture below the small circles are attachment points.  You can use confetti, soap flakes ( I think), or paper holes from a hole punch, sparkly pieces.  But they will flutter and get underfoot even if the blanket is far upstage or downstage.