[Chris Saper]

Sophie,

The innner, hidden mat will work great, as long as you are using acid free material. I often just tape strips of acid free board (with acid free tape) on each side of the mat, just a little narrower.

For the outer mat, get the Crescent black core boards; even if a little dust falls, it won't show on the beveled black edge to nearly the degree it would on a white core board.

Alternatively, if you want the look of an unmatted framed piece, you can buy long strips of extruded plastic spacers (a framer's supply). They have adhesive on one edge, and you can just cut them to the right size. They go betweeen the glass and the image.