Lightweight white chipboard matchboxes. The boxes fit pretty snugly in the "sleeves" so I'd suggest lightweight paper (or paint) to decorate the outside of the inner box (so you don't add much more thickness).
Small matchbox shrine by Kristin Batsel with an image from the Tinted Beauties Collage Sheet
Packaged in a corrugated cardboard box so they won't get crushed in transit.
INSIDE dimensions: Approx. 2-1/8" long x 1-1/8" wide x 5/8" deep.
Also shown with a standard Diamond matchbox for size comparison.
Set of 6 matchboxes.