Anyone who spends much time in the woods will often come across discarded shotgun cartridges. Whilst disposing of these, keep a few back and recycle them into a waterproof matchbox.
Obviously it should go without saying that this should only be done with used cartridges.
Take your first cartridge and heat the brass gently over a flame until the plastic starts to droop. Carefully grip the brass using something like pliers or a fire/heat proof glove (I used a rag soaked in water) and pull away from the plastic.
While you leave this to cool take the other cartridge and using a knife or small saw cut off the crimped section. Very carefully using a knife or possibly some very fine sand paper you can round off the blunt edge you have left on the plastic when you cut it – this will make slotting the two together easier.
And you are done – easy, free, recycling.