Hi,
It's a bit old topic, but...
I'm thinking about another way to slot a handle. Actually, it's another approach at all.
Let's make handle as two half's, and insert thin piece of veneer between them. Veneer can be shaped exactly as back side of the blade, so it fit perfectly to any shape (flat, curved...)
Also you can easily fit thickness of veneer to be exactly as thickness of the blade.
Something like this:
or this
You can also combine wood of different color if you prefer that kind of look.
First, cut both half's of the handle to rough shape, and drill holes in it. Then cut veneer to rough size, transfer shape of back side of the blade to veneer, cut it, and fine fit veneer to back side of the blade.
Now, temporary screw blade to one half of the handle, align veneer to blade and glue it in place. Veneer should be bit ticker than a blade.
After glue dries, you can machine the veneer to exact thickness of the blade, with plane, or something like this
Now, glue another half, aligning it with the screws.
BTW what you think of this kind of screws, with two holes for tightening, something like spanner on angle grinder? It can be fairly easily made on small lathe and drill press.
Another aproach
Another approach for cutting slot is to shim hand saw so it's exactly at the half of thickness of the handle, fix the saw and then cut slot by moving handle on top of the bench. Of course it wouldn't work for blades that have curved back. I find this trick on some other site, but can't remember where.
PS
Sory for my "Tarzan" English, it's not my native language.