Thanks to Cathy for this Frugal Baby Tip!
I made a wooden cradle when I was pregnant with Michael, and sewed the mattress, sheets and bumper pads for it. I just measured the inside of the cradle, added a 1/2 inch all round, and made it about 12" tall. Sew the fabric right sides together on three sides, insert padding and top-stitch all around. I added Velcro to hold it in place. I sewed one piece of Velcro on each bumper pad side and glued a matching piece on the walls of the cradle.