I fortuitously found a Nikon DK-21M eyepiece magnifier at Infomax in Market! Market! while looking for some computer parts. Which is a good thing because the computer-assembly exercise has turned out to be a disaster.
Anyway it was around $35 which is expensive as this is a sub-$30 item in the US, but one would have to deal with the hassle of shipping so it ends up about the same.
Here are three viewfinders: Canon EOS 350D, Pentax K20D without DK-21M, and K20D with the DK-21M. It's a usable magnification. Not quite a 5D-class viewfinder, but not too shabby.