I recalled seeing a sheet-type magnifier at Staples, and just searched for magnifier. Credit card and letter size are available.
These are a form of Fresnel lens. A simple lens is thick, but if you cut circles around the centre, you can collapse the lens to make it flat.
From experience, there is some magnification when the magnifier is laid flat on a page. More power when held a bit off the page.
For about $4, you can get a credit card sized unit and try it out. If too small, get the page size and cut the size you need from the centre of the sheet,