Coiling a long warp so that it pays out reliably can be a problem, especially on the occasions when you really need it not to tangle!
I don't coil the rope, but make it into a "daisy chain" (or caver's plait, or chain sennit) which is very easy to pay out in an emergency without tangling. Pictures and instructions here
That just gets stuffed into a canvas bag and I know that if I need to anchor in a hurry it will pay out without tangling. The downside is that it takes a while to daisy chain it all back up again, so may not be the best solution if you use the anchor often.
For frequent use then a spool might be a good option. I remember someone having a reel of flat webbing that seemed to work well and took up very little space. Not sure where you'd get such a thing though.