Loxie, My advice was going to be the same as Dan's, use a rasp, small triangle shaped file or just a utility knife and cut some grooves along the round parts just before the "corners" so the rush...
more I'd probably use the sharp edge of a file to create some "training" channels for the rush to sit in. Creating these small indentations gives the rush somewhere to sit until the next row snugs up...
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