I thought I read somewhere recently that one of the Ikea bowl set lids fits the Canon 300 or 500mm lenses..... the sets come in white or blue I think ---- sorry rather vague but may put you on the right track perhaps?
I replaced mine with a Schneider 120mm Push-On Lens Cap. It is available HERE. I know this is not a UK source, but it will arrive within 3-4 days. Compare that with WE (-express NOT) where I've got three orders on back-order. I should add, the Schneider lens cap could have been made for the 300/f2.8; the fit is perfect.
Discussion
3 repliesTIA
Tom
Mike
160mm cap
http://www.ikea.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10103&storeId=7&langId=-20&productId=11005