I believe the BOB trailers are very well designed, particularly the patented attachment method, so I didn't stray to far from their design.
I've included photo of the BOB Ibex trailer for comparison.
Step 1: Components and materials
Wheel: I was thinking of going with a 20" bmx wheel, but ended up using a 16" wheel like the BOB trailers. I found a few used kids bikes with 16" wheels, but went out and bought a new one for this build.
Shock: I found a coil rear bike shock on ebay from a moped dealer for about $13 including shipping. Would have liked to use an air shock, but they were more costly. I found some 1/2" axle bolts at my local Ace hardware store along with some nylon spacers.
Swingarm and rear triangle: I used an old BMX fork I had lying around as the swingarm for the rear wheel. The pivot is a steel bike hub the local bike shop gave me. Another fork was used for the vertical part of the rear triangle.
Frame: The frame is fabricated from 1/2" EMT (electrical metal tubing). 10' lengths from Home Depot cost $2 each. I used 4 with some waste and leftovers. CAUTIONARY NOTE: I use a flap wheel to remove galvanizing from the EMT before welding.
Miscellaneous: I used various pieces of steel plate (1/4" and 1/8") and steel angle and channel I had lying around from previous projects. Old spokes are used as retaining pins. Also used some scrap 1" EMT. Bottom was made from expanded steel.
BOB quick-release axle: This is the key to the Bob patented bike attachment. I probably could have made something similar, but I knew this was tried and tested. I got mine from Amazon for $26. There is a different version for use on bikes with solid rear axles.