The Squat Frame, or Trap Bar as it is often called, can be traced back to an old lifter named Leslie W. Carson. In 1969, Mr. Carson built and marketed a piece called “The Squat Frame” through the old Ironman magazine.
Mr. Carson offered both a wood and an all-metal version of the frame with two semi-circle notches on center that allowed you to use a barbell as the payload. The wood model cost about $15 each and the metal about $20. Ironman, under Perry Rader, offered exposure to low tech items like Carson's all the way up to the original Nautilus equipment.
The Gerhartz Trap Bar and knockoffs are Johnny-come-latelies, and their origins can be traced back to the original Carson squat apparatus.
Anyhow, you can make your own squat frame for leg work. Here is a design idea inspired by Carson's original model, and it too uses a barbell for the payload.