The system uses 3/4" steel conduit (.922 OD) and clamp materials purchased from Maker Pipe ( Look for the "Makerpipe" web site ). I was able to support 108 lbs with one of these assembled Bases and Cones, before the wooden bracket broke. I only plan on putting a 80 lb Kayak on it. Use good judgment if adding heavy items you your rack. This requires some conduit bending skills. Mine are OK, but good enough for my project. The white knob makes it so I can do a quick disconnect of the cross braces. I use a plastic floor trim (Home Depot/Lowes) and a part called Pole_Clamp to add support across the top (there are 2). The base part is designed for an 8mm T-Slot (Ebay Amazon). The problem is the "T" thickness is 5mm and the Retrax slot is 4mm. I belt sanded them down to fit. But You can also get the 6mm T-Slot bolts with 4mm thickness if you want. I also bought I-Bolts to aide in the tie-down capability. I needed longer 6mm machine screws and addition 6mm nuts in order to use the clamp knob on the Maker Pipe bracket hardware.
3D models in Fusion 360, Step and STL files available at...
https://grabcad.com/library/truck-rack-assembly-for-retrax-t-slots-1