I've got the same problem. I put it on the spare tyre when i was filling up the engine with oil. After about half an hour of driving there was this huge clunk, from what i later realised was the thing hitting something on it's way down. I'm broke so i can't afford a new one (something like US$10) (I'm in New Zealand if anybody wants to post me one for NZ$5).
Um, I don't know what the story with your guys engines are, i've been running it with the cap off for two weeks no problem, it's an ea81 engine. I had a rag stuck down it but it came out sometime over the last few days.
Anyway, the filler hole is 50ml wide, so i'm going shopping tonight for some baby food, from what i've seen they come in 50ml wide cans. I figure i could just cut the end off that and make the tabs and then stick a washer down the end.
Think it'll work?