I made one recently. Do a search with my user name. Basically, it's a threaded rod (I used M10), some nuts and a couple of large washers. As I had time on my hands then, I cut two sections of old steel handlebar, a short and a long piece. I MIG welded a captive nut into each end of the short section, drilled a hole in the middle of the long section and then welded the shaped short piece to the long piece to form a very short T shape. This gave me a handle to wind down the threaded rod rather than rely on a spanner. I then used a hole saw to cut two pieces of oak to face the washers with to protect the headset.
It worked very well.
