The best replacement for cob is adobe.

The best alternatives to cob are rated as following:
★★★★★  adobe
★★★  eob
★★★  eod
★★★  drake
★★★  walnut media
★★  straw bale
★★  briar
★★  smd
★★  cree
★  rammed earth
★  pony
★  cordwood
★  cob
★  gypsy vanner
★  earthbag
  cog
  smd led
  cobb
  dol
  batch
  walnut shell
  walnut
  strawbale
  popcorn
  buck

Your alternative for cob is not listed? Suggest!
Speaking about cob, one probably shouldn't forget about blanching.

Other search terms related to cob and their ratings are:
★★★★★ blanching
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Google Trends

Scale is based on the average worldwide traffic of cob in all years. Click on graph for more.
cob
adobe
eob