Lots of good suggestions here! But I've decided to stick the freehub in some wet clay to make the shape of the splines, then cut out some clay from around it, and also cut out a shape of a handle, then melt down one of the gold ingots I've got stashed up in the attic, and cast me a wrench. The main advantage is that gold is fairly soft metal and is not likely to mar the freehub.
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