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I have the Canon E-82U Lens Cap to cover my Canon EF 16-35 F/2.8 II L USM Lens - and to be honest it was a bit of a struggle initially to find a lens cover for this lens. When I bought the lens (second-hand) it came without a cap or a filter and I assumed it would be easy enough to find one to replace it......but I was quite wrong. However, eventually they appeared on Amazon and over the last 18 months or so have been much easier to get hold of. It cost me £8.99 from Amazon (including postage and packaging), but I note it is currently being sold on Amazon for £6.99. Both of these prices are okay because it's what you'd expect to pay for a Canon cover, but essentially it is just a mass produced bit of plastic and I would imagine that it costs a tiny fraction of this to produce.
The Canon E-82U lens cap is a stand alone cap cover for the designated lens - and so it fits the 88mm barrel perfectly. However, I don't think it fits any other lenses - but I could be wrong about this. It is made of plastic which is thick enough to be really sturdy and copes with being dropped and bashed about really well. It is black and so fits the colour of Canon S-SLR bodies and the colour of the lens also. It has the Canon logo on the front of the cap and as a Canon snob this is somewhat important to me!
Because it's made of fairly thick plastic, it's not as lightweight as some lens caps by other manufacturers - and it's also larger than many and so this obviously adds to the weight also. It has a place and click design and so it's just a matter of placing the lens can into position and then pushing it on until it clicks. The lens cap, once in position is securely fitted and do not come off accidentally, therefore providing good protection to the lens. It also fits well when I have a lens filter in place. To remove the lens cap it's just a matter of squeezing the two little squeezy bits together and the cap is removed cleanly.
Overall, I recommend this lens cap for the Canon EF 16-35 F/2.8 II L USM Lens. I think Sigma do one too, but I don't think it would be such a good fit.