An enlarger lens can be a great loupe. They are much smaller and lighter
than a standard lens, which makes a superb loupe. Enlarger lenses are
of course dirt cheap at the moment. The all plastic Schneider Componar-C
is to be had for $10! I used empty film cans as a base. Cut off the desired
length. If you are lucky they clip on or over or into one of the rings
of the lens. For the 80mm lens I simply cut a circular hole in the bottom
of the can.
For a groundglass loupe: a black Kodak can.
For a normal loupe: use a translucent Fuji can.
The strange finder on the left is my extreme sharpness checker. It is
an old viewfinder from a 8mm cine camera. It too rests on a film can base.
The image is upside down. But of course the right side up when I use it
on my groundglass!