Most of the ones I have seen are part of an aftermarket carrier.
Mine is one of the old BP/TReK Outfitter units:
The shrockworks ST carrier:
The Shrockworks ST carrier:
And a picture of a Rockware model:
I have seen pictures of people making thier own hilift mount on the joints but one thing you need to mind, as with the BP carrier, is the hilifts foot. I found out that if you bang something hard enough on your departure, the base of the hilift will take that shot as well depending on the angle and jam it upwards effecting whatever you have it attached to. There was a thread on this on the old Discus BBS but dont have the link handy at the moment.