I seem to only be able to use the headband magnifiers. They allow me to move my head and see things other than the fly, like when I use a bobbin threader or try to peel a certain number of fibers off a feather. I tried a big old magnifier but it didn't work well for me.
This is what I use and they are great: Head Strap Magnifier With Work Light
They have a 3x clip on magnifier, too: 3x Clip-On Magnifiers