Making these is much easier than I thought. Even if the slot is not parallel to the handle it still works fine. I suggest you make one or two in scrap wood to get confident with the process and to make sure it will fit your hand.
Scribe any existing handle onto paper or print the image below. Read my comment at the end about matching the hole to your hand. Allow for gloves if you wear them.
Cut out the ‘waste’ from your template leaving you with a positive image of your new handle and glue that to your wood with PVA or other water-soluble glue.
Drill holes at each turning point big enough to allow your blade to rotate. I prefer a bandsaw but a turning saw will also work.
If you can’t cut along the line cut down to it with a straight saw – which Slava Rode demonstrated at Sawfest 2018.
Now scribe an arc so your holes can line up. Eyeballing this is fine.
Drill holes along the arc just scribed. Better if you have variable-size drill bits. Forstner bits work well for me as you see below.
Now scribe a mid-point line where the slot will go. I used a marking gauge and you may devise some better jig to mark that mid-point.
Now you cut the slot. You only get one chance here so do have some practice attempts in scrapwood. I made the right-angle jig below which guarantees a perfectly vertical cut. But when cutting this handle my aim was poor and the slot drifted from left to right. Next time I won’t do it freehand and I will use the bandsaw fence. Other methods with a horizontal cut are discussed here. Slava does his freehand – he tells me with a 50% failure rate.
Do a test-fit. I decided to deepen the slot after this photo.
I was dismayed sawplate was not parallel to the handle. And I kept going to see if an offline handle mattered in use. It did not ☺
The slot can be made wider by putting two ripsaws together to nearly double the kerf, or filing rip teeth into some scrap steel.
Use rasps, knifes, spokeshaves, routers, sandpaper to shape the handle to your hand.
Do measure your hand’s width before you cut out your paper template and make sure your drilling reflects that width. My fingers are 92 mm wide and my drilling gives me that width.
The first one took quite a while but I then got into the swing of it. After making the ones below I soaked them in warm water and scraped off the paper template and glue.
Fixing the plate in the handle with set screws I will cover separately.