Last winter when I was trying to locate some gypsum for my wild-simulated plantings I went to a county farmers co-op over in a county not too far from my home.
I found that they had some hoe-heads in their tools dept that were somewhat like my old digger and accepted the (smaller round handle) instead of that wide and heavy handle that most mattox type tools require.
I think that is the key to getting one that is strong, but light. It has to be built for accepting that round regular hoe type handle.
When you do put a handle in it, don't use ash or birch (what they usually come with) but get you a good stout hickory or oak handle (something that will take the pressure).
Anyway they had two of these brand new hanging on a nail and I bought both.
I put a long birch handle in the one on the left and use it in my garden.
The one on the right I worked over (with my big shop grinder wheel) cutting it down to make it more like what I wanted in a seng digger. I have not finished it yet. Still have one more bit to take off on the top right (notice the black line I have marked).
Once I get that part cut off - it will have a 2\" wide blade on one end that is a lil over 6\" long, and the other side is 1 1/4 inch wide by a lil over 6 inch long and tapers down to be a bit more pointed.
I think this will make a good seng digger and I am mostly getting it ready for a spare or for a friend to take along. Some times I go and dig on other folks property, and show them how to find it, teach them good stewardship, how to plant wild simulated, etc - and it's good to have a spare digger for them to use.
Couple of pics showing what I found.
You might try looking around at local farmers co-op's if you all have those up north there.