Nope I have no idea who Don Zeck is, but he does a fine line in lens caps for big glass. The Canon supplied lens cap for the 400mm F2.8 is basically a Nose Bag, a right pain in the arse to use and only properly fits when the Lenshood is reversed. OK most of the time not a problem, but when it is not fitted it takes up an unnecessary amount of space. It won’t fit in your pocket!
Anyhoo only available to order from the US, and not exactly cheap (although cheaper than the Canon replacement) it arrived in about 15 days, which compared to the waiting time from stuff from China is not too bad. Just have to make sure that I don’t misplace this one.