Don't bother buying a fid: For the size of lines we work with you probably have all the needed parts in your toolbox already.

Pick a length of thin wire (60cms or 2 foot), bend it in two in the middle, twist the ends together, cover the twist with tape and slide a plastic ball to cover the tape. If you want to be really fancy use stainless steel wire and bind the two ends with a nicropress sleeve instead of just twisting.

That's all you need to do all the splices you will ever need to do on your cat.