I bought two extra TL-21's to make fixtures.
On one, I replaced the two black cables with two teflon shielded twisted pair cables to Kelvin clips, roughly 60 cm long.
On the other, I replaced the black cables with four RG-174 coax cables to BNC plugs, to allow making "black box" fixtures with binding posts at different spacings.
In the black-box fixtures, I included a small switch to short the binding posts together for short-circuit calibration, using multiple-pole miniature toggles to reduce the contact resistance.
I kept the original TL-21: I haven't had any problems with the alligators except that the plastic covers make them stiff.
I'm looking for a good way to keep the alligators the correct distance apart (corresponding to the final spacing on the component) when doing the open-circuit calibration.