The best replacement for boron is borax.

The best alternatives to boron are rated as following:
★★★★★  borax
★★★  boric acid
★  guyton
★  permethrin
★  rio hondo prep
★  bishop
★  borate
★  nickel teflon
★  hardened steel
  fail zero
  chrome bcg
  costanzo
  np3
  phosphate
  salesian
  melonite

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Speaking about boron, one probably shouldn't forget about calcium.

Other search terms related to boron and their ratings are:
★★★★★ calcium
★ silicon
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boron
borax
boric acid