I know perspex can be expensive but I'd have thought someone local could get you a piece of glass the right size not too expensively. After all, a replacement pane in a single glazed window isn't that expensive. Maybe try a local hardware store or phone a local glazier?
Or this place online does perspex and for those measurements it works out at £29.13 (1080mm x 500mm x 50mm) which is the mm equivalent of cm. Not sure what the postage would be though.
Clear 000 Perspex Sheet Disc Shape | Cut To Size