Originally Posted by rustguard

A small piece of angle iron would be one way (remember to always use soft jaws to clamp threads). Or even just clamp your forks low down in a vice and rest something a little less than half the diameter of the forks in the groove above the threaded area, a small peice of rod sat in the edge will keep it sitting at 90 degrees.
When undertaking a project you have 2 choices, tool up for production or improvise with what you have. If you want to do a decent job of this you need some tooling, like a good size vice, otherwise it is just bush mechanics. If you take your time, concentrate and do a basic setup you can do a good job. A file is just a cutting tool but powered by your arm instead of an engine.

Ive been taught to file flat and square to 0.02 of a millimeter and I have the papers to prove it