If you are really down to your last sen, you can cut thin ( half inch or less ) strips out of your plastic and stretch them across the board, fastening them only around the ends of the backing board and where they fold around to the rear.
22.
The treatment of the back of the framed artwork varies widely, from usually nothing in the case of oils, to the frequent use of foam-core boards ( available in archival acid free versions both in black and white ) and other backing boards to provide support, or backing paper or " dust covers " to keep dust and insects out.
