Elastic band and a bread bag for now mate (make sure it's crumbfree!). Then get down the scrapper, the stealers, or onto VAGParts and get a new one ASAP.
A bloke I used to work with topped up the oil on one of our LT35's, forgot to put the cap back on, warmed the van up and did 6 miles down the road before he noticed the smell as the oil splashed out through the filler. I don't know how much he lost, but I doubt that engine bay will ever have a rust problem
