Many moons ago, Canon made an "Eyepiece Magnifier S" with a couple of adapters for the different bodies at the time but probably long since vanished.
As suggested above, a new focus screen would be a more normal option nowadays....a split prism with matte to match up with the apertures of your intended lenses.
Bob
1Dx2 (2), 5DSR, 1Ds3, 1D4, 5D2(590nm), 5D2(720nm) EF600 EF400 EF300-II EF300 EF200 EF200-II EF180L EF135L EF100 EF85-II EF50L TS-E17/4 TS-E24L-II TS-E45 TS-E90 MP-E65 EF70-200-II EF24-70/2.8-II EF16-35/4 EF8-15/4 EF11-24/4 Zeiss 15/2.8 21/2.8 25/2 28/2 35/1.4 35/2 50/2 85/1.4 100/2 135/2 T/C's L-SC & a WIFE!