I carry one or the other of these on my keychains:

http://www.allmed.net/catalog/item/87/966

http://www.allmed.net/catalog/item/87/144


My 2 cents worth on air casts:
- easy to use
- effective
- bulky
- prone to cracking if stored in high heat or cold and not used for a long time
- I wouldn’t try walking very far with one on (not very durable used this way and bulk beneath foot would make walking interesting)

I would second using a SAM splint (make sure you have an appropriate ACE wrap or equivalent) (much more durable to walk on and much less bulk under foot)
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"Sometimes, it's better to be lucky than skillfull"