I bought a couple of the Hultafors heavy duty craftsman's knives from an online trade supplier . Two knives and postage only cost just over eleven quid so its worth shopping around rather than going straight to Amazon. I had the intention of customizing them both but decided to keep one standard as the plastic handle is hygienic for preparing game at home.
The other (bottom in the picture) one represents my second (the second I'll admit to anyway
) attempt at making a knife handle(my first attempt is the holly one on the top knife) I used a piece of ash with antler ends. I fitted a copper disc where the handle meets the blade as the antler was a bit scruffy and some old leather from a pair of site boots in between each section. I had all the bits laying around as I'm a bit of a hoarder so its not brilliant cosmetically . Basically a walk in the woods should provide you with something to work with .
Sawing and chipping the plastic handle off wasn't too hard . The tang does not reach the full length of the plastic handle so I welded a bolt onto it and cut the head off. The metal is good and welds easily with out affecting the temper of the blade. Probably not essential but having a full length tang is good for peace of mind and the thread means you can use a nut to wind the handle on tight and then saw the the excess thread off when the glue used to hold the handle on has set . I used two part boat building epoxy as glue simply because I had some in the shed but I'm sure a decent quality Araldite-type glue would do .
No way am I an expert but at least you have one answer to your post which might provoke those with more knowledge to chip in .
All the best