Actually, I found some Flecktarn camo tape on Amazon for £9.
http://www.amazon.co.uk/MFH-FLECKTARN-CAMO-FABRIC-TAPE/dp/B003WJEFL6/ref=sr_1_1?ie=UTF8&qid=1362231405&sr=8-1
The woodland camo tape on my gun is actually latex, so it sticks to itself and not the gun, so I can just slide it off when I have to.
Talking about your gun sleeve idea, I actually like the idea of that and I might try that. I actually have a Flecktarn shirt in my wardrobe somewhere, so I'll use that but thanks for offering