Hi Sodo,
I eventually built what I call a "banana" bucket. It is a narrow, open sided, European designed bucket and works great for my soil (clay, no rocks). It is about 7" wide.
A TBN member from the Netherlands gave me the design. The open sides keeps damp clay from sticking in the bucket. When digging a trench the trench walls keeps the bucket full. In the spring and early summer (best time for digging clay anyway) the bucket works exceeding well dumping heaping full buckets. Later in the year, as the clay drys out, it can be somewhat of a balancing act keeping the flat bottom of the bucket horizontal to keep the bucket semi full when exiting the trench and swinging to dump.
The cutting edge is just mild steel sharpened at about 45 degrees. It has worn down a bit after 2000 ft or so of trenching but I left space for resharpening. I did not need to add teeth although removable teeth (2) may have helped one late summer, deep, hard-as-a-rock clay trenching project.
Here's a photo on my standard and banana buckets.
