Go get some "batting" from your favorite yarn and craft store like Joanne's Fabrics or Michael's. it costs ~ a buck a yard. Sometimes they sell it by the pound as well. Real cheap.
This is synthetic fluffy fibers used for stuffing pillows n such, filling comforters etc. Real cheap, and makes a great filter when compressed in the space between the rain fly and tent roof.
Best thing is, you don't need to glue, sew, tape or nothing. just jam it in there tight.
Next, cover the inside of your tent with a cheap king size bet sheet. Before you turn in for the night/morning, take out the sheet and shake out the dust, fold and bring back into the tent.