I used copper wire to make a lineset. It bends back onto itself nicely without breaking, but is plenty stiff to push or pull through as you are making your splices. I rounded off the ends of the wire to prevent it catching on strands.
As others have said, definitely spend some time practicing before you make anything you plan to use. It's not hard, but if you're doing something like the brummel with a bury, there are several steps you have to get right. Learning how to make everything consistent, especially the taper, is important.
Can you make the Brummel splice with a guitar chord instead of the fid?
Yes. You don't need a fid. You will want a pen/pencil/something similar, preferably with a sharp but rounded end to nicely slide thru the dyneema so each side has roughly equal strands. Obviously a pencil is not working with a 1.5mm thick dyneema flying line tho....You don't have to be perfect, but the thicker the dyneema the easier you can separate the strands perfectly. A nice center hole thru the dyneema is what you are looking for to feed the end of the dyneema thru, however you do this is fine.
I bought the below hollow stainless steel knitting needles for $15 and they cover any size dyneema you are working with. I use a mix of them and some really small diameter violin string I somehow acquired. The problem with using fids is with the small diameters we work with it is hard to get the end of the dyneema into the fid. I end up running my fid thru the dyeema to widen it and straighten it and then the violin string goes thru perfect.
If I was anyone just doing splicing 1x per year I would just use guitar string...so easy.