Hi There Toofy
The short answer is take a 4mm rivet hold the thick end in a pair of pliers and tap the long shaft on a bench on anything and the shaft will come out and presto zezto a string ferule
your welcome and you can use the shaft as a bridge of nut set into an appropriate piece of wood if that makes sense.....
where are you based in the UK
Another alternative Toofy,is bicycle spoke nipples,you can get them in many colours and cheap
I use 3mm rivets from tool station nice and cheap.