lf-photo.org.uk

Lowepro to several pouches including a waterbottle holder in their Street and Field Range. Don't know whether or not you can attach them to your bag though.

If you give Fotospeed a ring you can try/collect the lens cap from them in Corsham. I found them very helpful!

B+W do a slim 72mm push on cap - B+W SLIM Front Lens Cap 305 - (as supplied with their slim filters), but it will probably be a special order from RW or somewhere.