Four options:
1. Make a key from a key you have nicked, using metal you have lying about, melted in a portable furnace and cast in a sand mould you have just cobbled up from kiln sand you just happen to have lying about. Two minor snags. Power source for the furnace and heating the sand mould up to kin hot so you don't blast hot metal every where when pouring into a cold wet mould.
2. Cut a key from a key you have nicked using your own key cutter. 1 snag power source.
3. Have a key cut at "Timsons" from a key you have nicked.
4.My favourite. Nick the key, stick it in the lock and turn it thus avoiding all the other faffing about.
I suppose you could use the greasy chin and baccy tin full of Plasticine trick a la James Fox aka The Jackal right in front of the receptionist otherwise its back to from a key you have nicked
Buy a cheapo kit from the internet. £20 or so.
Make your own mold using foundry sand well rammed into a sturdy box. Two halves. You will need an inner and an outer for the metal to be poured in and to allow a little overfill. Also a strap to hold the two halves together
Melt the low temperature metal (tin alloy?) in a pan or crucible on the cooker, until it is runny like water. Scrape off the film on top and pour into the mold. Gently tap the mold to get rid of bubbles ... then leave to cool.
When cool enough, tapm the mold again, open the two halves and tip out the key copy.
It will likely be a bit rough around the edges but using a file, or grinding wheel fettle the edge.
Voila a new key
Easy peasy
Thousands of youngsters and older guys have made toy soldiers etc using a very similar method
As a kid during the war, I saw my friend making dozens of soldiers, model planes etc using this simple method.
I think he used lead, but he was only a school boy and found it very straight forward.
What ever metal is used must have a low melting point and have reasonably good tensile strength for hobby casting of a key. It was only for using once.
There are a number of instructive videos on the internet.
It is the professional and specialist casting that needs furnaces and really high temperatures. Now that is a different kettle of fish. The metal then is spitting white hot flecks of metal as it is poured. The pourers wear leather spats over their lower legs, eye protectors and thick leather gauntlets