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I reckon a cheap v-brake booster would work for that. As a replacement for the Dia Compe keirin brake platform.
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I've got some of those on an old bike. They work fine for their intended use so I'm sure they'd be fine as a basket mount. Quite sturdy.
They can be had much cheaper on AliExpress though (obviously you have to wait for it to arrive from China though). Think I paid about £2 each for mine. But it was a few years ago.
I think what you guys are talking about has been done on this thread by flatik
https://www.lfgss.com/comments/16203693/