The fork was cleaned up and squeezed down to fit a child bike wheel. The fork originally came off a 70's era 10 speed.
To be safe I drilled some holes in the end of the fork and added some axle retainers (optional).
You will see from the picture I laid it out with 3/4" EMT to start (single tube). It was WAY to flexible so I added a second row of EMT and it worked perfectly. In hind sight I would have used two rows of 3/4" vs. the 1/2" and 3/4" combo. I also would have borrowed a bender and saved myself the welding. The dimensions will vary depending on your bike, fork and the wheel size. I eyeballed it to the frame so that it had generous wheel clearance for turning and camber changes. My goal was to keep the floor of the trailer level at rest. Go low so it handles well, but not so low it hits all the time. Mine is a little over 4" from the ground to it's lowest point.
If you had a tubing bender and new pipe this would be snap but I had to weld all the junctions together.