I made mine out of 1/8" aluminum plate. I simply scibbed the shape (you could even trace it off the cap and cut slightly smaller), drilled a small hole at each corner and used my dremel to cut between the holes. The whole process took about 1 hour including handfiling to clean up edges. It is a little crude, but I was in a hurry and needed the tool.
Always planned on making another a little cleaner, but the first one works perfect and have had trouble justifying spending any time at it.
I would suggest the aluminum though, it's much easier to work with and there is almost no chance of it damaging the fork, as it is softer than the actual nut. There is very little torque to contend with so the aluminum one is more than strong enough. It also does not really need to be highly precision either, so don't panic if you make a small booboo.
Good luck