Nothing special my friend , I just cut and shape them as I need them .I have a dozen of them ,long, short, wider ...etc . I use these two the most time , I even put handles on them . Two minute job for that slot .Start one side and then flip broach and use that slot as guide for slot on other side ....then finish both side
I make them from this saws.....teeth are from M42 steel , will last lifetime in my shop .
Thank you, @Natlek I felt almost certain that you'd have made your own, and wanted to see not only the material and construction but shape. Simply put, you're more creative than me with tools. I know how to make pharmaceuticals, poison poeple, blow shit up, etc, as a chemist, but mechanical comes with effort. And those are nice slots.

I use the same technique as you with the hole drilled first with a bit thick enough not to wander. I used a lot of stag, so used a 3/8" (9.5mm

) bit to get the spongey core out, then slotted.
I have a purchased broach from John Perry and one David Wesner (RIP) made and gifted me.
I have a couple I've made from jigsaw blades but they're too short for much use other than the specific reason I ground them.
There's a little bag of spent reciprocating saw blades somewhere in the shop, so I will try to use your examples as ideas. Thank, you, sir! It's appreciated. I haven't made any full tang knives in a long time, so it is good to see what people are using now for digging the holes. 12 years away has produced a lot of change.
Thanks, again, my friend.