I did a Bing search for pictures of a thumb and started designing a type that looked like it was simple and workable. Your bx25 has a more curved hoe arm than my bx23, which are more straight. I used two pieces of 1/4 plate that cinch up on the hoe frame. It can be moved up or down depending on need. It did take a bit of testing and reset to get what I wanted. Hence the additional holes in the mount plate. The test was when I picked up a 24" basalt rock, tucked it in and drove off with it. I read a posting here on this site last year where a fellow said "if you don't have a thumb on your hoe, you are missing out on half of what your hoe can do". Was he ever right. My thumb works well and saved 3-4 hundred dollars that a bought one is.
My tools were a drill press, milwaukee porta-band, welder, grinder.
Richard