Half-fluted reamers are easy enough to make. Grind or file a metal rod to the proper taper, say, by chucking it in the drill press. Once you've got that, file off a little less than half the diameter to make a 'D' shaped cross section. This cuts by a scraping action. If you're not doing too many holes you can make one from aluminum that will work fine. Mild steel will go for quite some time. I made one from drill rod more than thirty years ago that's still going strong.
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