I've found that one of those really nasty, cheap laundry bags does the job well enough.
Having the anchor and warp bagged up makes it easier to deploy with no tangles:
- make a hole in the bottom of the bag and feed a couple of metres of warp through from the inside (this is your "bitter end")
- lay the rest of the warp into the bag without making any attempt to coil it -- just let it go in as it wants to
- put the anchor into the bag last, then tie round the whole lot using the bit that you pushed through the bottom of the bag
This gives you a compact and easy to manage package. When you need to use the anchor, untie the bundle, attach the end to something secure, then pay out the anchor line straight from the bag. Make it off in whatever way you prefer.
The bag may only last a season before it falls apart, but any anchor bag is going to take a beating on a RIB. Have a look in your pound shop before you spend more!