An example of a reasonably priced, UK sourced filter meant for the purpose - from memory though it may not be wide enough for the display on the Maplin system - probably meant to cover the four or six digits of a typical digital clock project.
https://www.bitsbox.co.uk/index.php?...oducts_id=2686
If you're a good craftsman you could butt two of them up against each other to cover the whole display with the join on the boundary between two display cells, or perhaps contact the supplier to see if they can source a longer window?
(I have not used this supplier myself).