A couple quick comments.
The white plexiglass that Aggie describes is a common material in commercial backlit signs. Any medium to large city will have a plastics supplier who will cut a piece to the size you want, saving you the cost of shipping.
If you are gong to bleach an entire print I have found that it is important to use a large tray and agitate very gently by moving the solution over the top of the print with brush while not touching the print. If you rock the tray you will get excessive bleaching in the corners of the print where the solution backflows from the corners of the tray.