Peter I round the edges of the tang, and forge it slightly undersize then if I was to do wood scales. I use leather slabs to add thickness, could use thin plastic or a regular coil wrap of stripped core parachord to add some beef. if you are lookng for that much increase in thickness, I would use some kydex slabs or uncored paracord.
I drill a hole in the end of the tang to tie my finish knot through, there's a lot of different finish knots you can do, experiment around but best to keep it as flat and low profile as you can for ergonomics.
Paracord is as soft as it is tight, in that I mean the tighter it is the harder it is. I use the big spring clamps that home depot sells for 1$ to help hold the wrap while my hands rest from pulling it as tight as possible. If you want to secure it 100% then after it is all wrapped and tied, coat it in super glue or clear epoxy. The japanese would sometimes Lacquer the handles, but tightness is the key here if there is any play it will just keep getting looser.