Upload & Sell: On
I have used the Aquatech cap on my current 300mm 2.8 non-IS, as well as my previous 300. Works a lot better than the Canon one, protects the lens well, and is easy to get on/off. Not to mention you can put it on and off with the hood in place. Great product IMO.
EDIT: I forgot to mention, if you buy a new one, as I did a few weeks ago, the "Aquatech" logo is no longer blue, it's black like the rest of it. Maybe that was by popular demand, no idea, but either way, if you get one and it doesn't look like the picture, you aren't the only one. Appears to be the same otherwise.