I've made a new and improved version!
Due to COVID19 related shortages I was asked if this could be done, so I designed one. Unbeknownst to me this was done before, so my design... differs a bit.
It should be scaled to about 40%-60% od the STL size, depending on the need and printbed size. It's designed to fit a sheet of A4 transparency film in landscape, which will need to be cut down to size depending on model scaling.
You'll also need to attach a string, rubber band or elastic to the holes on the sides. The tention from the string will keep the transparency film secure.