as simple a thing as cardboard with aluminum foil over it on the inside, tied in a triangular shape with string and rags stuffed in the end. Any small narrow box that will hold steam will do it. Don't make it any larger than you must for the pieces you intend to steam.
Got a pressure cooker? Got a camp stove? Put the pressure cooker full of water on the camp stove, leave off the weight, run a chunk of pvc from the outlet to the steam box and fire it off. Pieces that thin should steam in 20 minutes or less, but give'em 30. Tip - wear gloves to handle them afterward- they'll be HOT!! Good luck