Personally I wouldn't bother. For the cost of a new key you could take it to a decent tyre place, get them all removed and either replace with standard bolts or get an after market set. I'd put normal bolts on, they're cheap enough from a scrapyard and usually left lying around on the floor and let's face it, nobody is going to steal a set of standard Peugeot alloys.
The likely reason for the key missing is because like most keys that get used, it failed and either the pins got damaged or fell out or it damaged the locking bolts.
Are you certain it isn't there - it's normally found somewhere on the toolkit holder in the spare wheel. It may be somwhere on the side - not necessarily in the top.