I'm a knifemaker too....(Voting member of the Knifemaker's Guild since 1989)
I couldn't find a commercial trowel that held up either, or one that suited my digging style so I made one.
This one is two inches wide at it's widest point and 14 " long. Heh...I'm optimistic about finding deep targets.
Anyway, I ground this out of L6 tool steel, a little over 1/4" thick. It has a 10" hollowgrind on the top side, and two shallower 10" hollowgrinds on the bottom. I left close to full thickness for a strip in the middle for additional strength. The shaft has a fuller (groove) ground in for additional strength as well...kind of like an I-beam.
The handle has been taper ground toward the end to preserve strength and reduce weight. Handle material is green canvas Micarta with nickel-silver bolts, and a brass lined lanyard hole.
I tapered the digging end of this trowel towards the tip so it slides nicely into the dirt. The edges are sharp to cut sod.
The key to this unit lasting is the heat treatment. Its L6 heat treated to Bainite...which basically means you can pry with it and bend it up to about 45 degrees (which I've never been able to actually do...) and it will spring back true....it won't break until you put a long pipe on the end and bend it 90 to 130 degrees off of 'straight'. The edges of the trowel portion are hardened and the rest of the steel is heat treated the high 40's to low 50's, Rockwell scale (a very strong spring temper).
My prototype last year worked VERY well, and that wasn't even the same steel or careful heat treatment. I just wanted to try out the design. Then I went all out with the L6 bainite heat treatment. I don't expect I'll live long enough to wear it out or break it! Now if I just don't leave it laying in a park somewhere
I used to use a knife to cut plugs, but this has replaced them. Also, if one ever needed to, it makes a great hatchet for roots ( or self defense, if need be!

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Knipper