I'm working here on an embroidery hoop, but this technique works on any kind of framed work. Here's what the end result looks like.
Framed art work
Brown kraft paper (light to medium weight seems to work best)
Spray water bottle
Xacto knife or craft cutter
The photos aren't the best - I HAD to choose to do this on the most dreary, rainy day here so I had very little natural light coming in the window - but I think you can get the idea.
1) First cut a piece of kraft paper slightly larger than your artwork.
2) Place kraft glue around the edge of the frame. Almost any kind of kraft glue will work.
3) Smooth and secure the kraft paper over the glue.
4) Place the artwork flat, kraft paper side down, and allow it to dry thoroughly.
5) Once the piece is completely dry, carefully trim the paper close around the artwork.
6) Here's where you take a little leap of faith. Lightly and carefully spritz the kraft paper with water.
7) Don't saturate the paper; just a light spritz all around will do. Let dry.
9) And there you have it - a nice, neat dust cover to protect your beautiful artwork!