I know that somewhere I saw an A4 size magnifiying sheet. If you could prop up something like that and then work behind the 'window', that ought to do it. No idea where or how though. Aha - google 'magnifying sheet UK'. Some of the items look hopeful.
Sadly these magnifiers are 'Fresnel magnifiers' which means that detail is not good. It makes the overall thing look bigger, but less clear. Ok if you can cope withthe added blur, but for fine sewing it is a no no...
Fresnel means that you have rings of prisms, which behave likr a lens, but the lines cause interference.