Genuine Unistrut P2950-EG 4 Wheel Trolley Assembly for use with P1000, P1001, P5000, P5001, P5500, P5501 and All 1-5/8 or Taller Strut Channel by UNISTRUT : Amazon.ca: Industrial & Scientific
How to make strut-channel (Unistrut) trolleys from old door hinges - YouTube