Nikon 200-400mm and aquatech front cap
For those of you who might be interested in putting a cap in front of that lens:
I bought and received the ASCN-3 Aquatech soft cap for the Nikon 300mm f/2.8 and 200-400mm f/4. It fits the lens very tightly and actually makes an airtight seal as it bulges slightly from the center. Once it is in place, there is no way any dust or debris will get inside. The cap will not fall off if the lens is held upside down. The only way of removing it is by pulling on the top lip as seen in this video:
I like it much better than Nikon's padded sheath but I have 2 concerns: Contrary to what the guy in the video from Aquatech says, it does touch the protective glass on the extreme perimeter (I have a circular smudge there that proves it). It probably doesn't touch any glass surface if the protective glass is removed. The smudge can probably be removed easily. Also, time will tell if it's a durable solution. It can be found at B&H.