How rustic would you like them to look?
I use willow for file handles = drill a hole and drop in the tapered end, a few hard taps on the bench = done.
I build some wood carving tools, up to an 18" handle.
For the planer knife, I cut a sloppy tang slot, gooped it
full of some sort of epoxy metal (was black color) and pushed the tang into that. Forced a strip of wood into each side of the tang slot and cleaned up the squeeze-out the next day.
I level split slabs of western red cedar with that knife. It follows the slight curves which a hand plane can't do. No sign whatsoever of it letting go.
Really bang and scuff up the tang parts. Needs to be some
"tooth" to sit in the filler. Clamp in the vise and get busy with a big coarse file.
I'd drill the hole just a little bit bigger than the tang.
Fill 1/2 with, say, epoxy, and ram in the metal.