Martha Stewart Living, June 2005
Begin with 8 lengths of bead board. With miter box and saw, cut 4 lengths to 19 inches and 4 to 26 inches, cutting ends at a 45-degree angle. Fit pairs of slats together (one shorter, one longer) to form frame sides.
Using brads, affix 2 sides of the frame to plywood, facing up. Slip mirror into grooves on those 2 sides, then place the remaining 2 sides, fitting mirror into grooves; secure with brads. Set brads, and fill holes with wood putty; paint.