How thick is the intended diameter in Hammer units? It seems like you're making a relatively small arch if a 20-hu thick arch isn't making any negative space.
Hammer has difficulty making arches that are very tiny as it has to keep each part on-grid, leading to some weird forms at smaller grids. I'm guessing this arch is around 16 units or so in diameter? What do you need such a tiny set of arch brushes for?
I haven't seen the thing you're trying to detail but it may just be insignificant enough of a detail that you can just eyeball it with maybe 3 little brushes yourself or just substitute it with a similar prop.
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