acetate sheet (or similar)for a picture frame 55 cms x 45cms (Fords Farm RG31)

A LONG shot this. I only need one sheet of lightweight acetate ( or similar) for this picture frame but, looking on the internet to purchase, I either have to buy 10 sheets or buy cut to size with a minimum order of anything from £15 to £50, often plus postage! Glass would be too heavy for this lightweight frame, so anything suitable and bigger than 55cms x 45cms would be acceptable. Thank you.

Because the frame is leaning forward on its string the backing card is bowing slightlyand slipping out of the top of the frame, allowing the photos to move. Considered buying a whole new frame but it was a gift and the photos have been trimmed to fit this frame.