Do a search for Bro-Tek. Richard is a TBN member that fabricated a bolt on thumb for his BX, then started fabricating and selling them through a website he created.
There's a summary of the two most popular thumbs with links to the most relevant TBN threads on the BXtractors.info Thumb page. Also, at least two TBN members have fabricated their own thumbs for a BX.