I know that you're looking to buy a higher-end commercial product, but I was able to make something that I find much easier to use than any loupe I've ever had. I took a +4 close-up lens from a set that was fairly large and attached it (with simple tape) to the end of a plastic tube cut to the focal length of that lens by itself (rough measure by focusing on fine print while sliding the lens along a ruler). The plastic tube I used was cut from a tube of painter's caulk I had just used up. The plastic is soft enough to put flat against the GG without worry and the lens shows a considerable enlargement of the screen without my having to put my eye down to view it–helps a lot with these progressive lens glasses I wear! It's a bit bulky to carry around, and a string around my neck isn't really very comfortable, but I keep it in the case with the 8x10 and have it there whenever I set up. Close-up diopters are a dime a dozen at most photo shows and the tube was free. Having one in each camera case is a pretty easy solution.

Just sayin'.