The cheapest ultra-high-quality option is an old 35mm or 50mm lens. There are plenty of f/2 lenses in defunct mounts that can be had for $10-20 that will be as sharp or sharper than any loupe you can buy. The main drawback is that they're pretty big and heavy. Once you have one you're happy with, you can glue a bit of PVC pipe to the back to hold it about the right distance from the GG.