Lens cap - kind of

[Winston Churchill (Sir)]

use a plastic CD case to make a protective cover for your lens - obviously a bit more bulky but if you are in a jam having lost your lens cap - here we go
This fits my 180-600 lens perfectly

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[Winston Churchill (Sir)]

here's another idea

 

[Hooligans Imagery]

Never cut a hole in anything that can be an emergency coffee cup. Words to caffeinate by.

 

[JDG Photo]

I use this: The LCF-740E Soft Cap from Sigma is a spare or replacement lens cap made to fit the Sigma 150-600mm f/5-6.3 DG OS HSM Sports Lens. The lens cap is designed to protect the front element of the lens from scratches or damage. The diameter of the cap is approximately 105mm.