[Magpie]

Easiest home made way I find is to get some bungie cord (any chandlers will have it in lots of diameters, get something around 3mm. Tie a simple granny knot in the bungie with your gun line through it and pull it REALLY tight with a pair of pliers on the tag end. Now rap the gun line around the bungie for about a foot and tie another overhand knot in the bungie around the line, again, TIGHT.

The two knots lock the coils around the bungie but the coils allow the bungie to stretch whilst still having all the strength of your mainline holding the fish.