I decided to take another crack at this, found a wallpaper paste brush with 1 3/8" x 7/8" x 6" of body plus the handle. There's a bunch of manufacturers of these, so it;s just a matter of finding one with enough meat to cut down. Since we don't have actual measurements I figured this was close enough. To my eye is looks like a D shape rather than a true semi-circle. Unfortunately these only come with natural color Tampico bristles so I had to dye them, Rit dye didn't go quite as dark as I'd like but it is darker in person than in the photo. Final dimensions are 3 5/8" x 7/8" x 1 3/8", with 3 1/4" bristles.
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