Hi custard,
Last year I bought some un-handled carving gouges from Ashley Isles, at the Northern Woodworking Show, along with some un-drilled handles. When I asked advice about fitting the handles I was told to drill a hole, no taper, no steps, the same diameter as the across flats dimension of the square tang. Then just drive the handle onto the blade, using the bolster on the tang resting on the vice jaws for support. Now the Ashley Isles tangs do not have a course taper and this may make a difference. With some apprehension I followed the advice and had no problems at all, thank goodness.
Of course if you are nervous you could always make the hole an easy fit on the tang and apply Araldite, or whatever the name is these days. But of course that is sort of cheating isn't it?
In terms of ferrules, does copper tube work? Readily available in 15mm and 22mm.
xy