I recommend measuring the space where the fretwork panels will go and cutting according to your measurements. Measure, mark and cut X pieces, mitering the ends at a 45º angle. Instead of using a tape measure, it is best to hold the boards in place, mark and cut to size to ensure the most precise fit.
Next, cut the 8” pieces, mitering the ends at a 45º angle, not parallel, then attach to the X pieces with wood glue. Clamp and let dry.
Optional: toenail 1-1/4” brad nails to secure the pieces together. Toenailing is simply driving a nail at an angle through a board.