Not my idea and I haven't tried it yet however this looks like a great idea from a UK Mould Maker JT-Bullet-Moulds:
He uses a spent CO2/NO canister from a soda-maker or cream whipper, or the larger canisters from a bicycle tyre inflator:
There is a pdf of his instructions here:
http://jt-bullet-moulds.co.uk/docs/Lead_Dipper.pdf
I'm going to whip one up just to try it.