12" bike wheel (kid's front wheel)
20" front fork (off of a cheep kid's BMX bike)
8" Hoss wheel hoe stirrup (comes with nuts and bolts, $30)
4' piece of 1" EMT bent at ~135deg angle (bend to fit)
2 self tapping roofing screws
1" stem and handle bars
Drill 2 holes as high up on the front fork as needed to match the spacing in the stirrup. The further from the wheel, the better. More room between the blade and wheel means less chance for weeds to get bound up behind the wheel and better leverage on the cutting blade.
Bend EMT to angle using a conduit bender of what you will. (Find an electrician to help bend this for you, or if you are picking up scrap EMT, ask the folks at the hardware store to bend it for you. Or make a pipe bender with scrap 2x4s and a bike rim bolted to a picnic table) Ours is bend to about 135 deg. angle. The important thing is that your force in pushing goes into the blade - and not straight at the wheel or at a point behind the blade, causing gouging.
Slide pipe over the steering tube of the fork with bend as close to the end of the steering tube as possible. You may have to cut the end of the steering tube to fit. Make sure the fork and EMT pipe are in line.
Screw steer tube to EMT w self tapping roofing screws in 2 locations.
Slide the stem w handle bars into the pipe. Adjust the handle bars to fit. Carry the tool (allen key) to adjust handle bars with you, as different users will want to adjust the handles differently. We like drop bars because they offer the most options, but other styles should work well too. I'd suggest attaching an allen key to the EMT pipe with a piece of cut inner tube rubber band, so it's always at hand.