As it happens:
I put together a basic kit in an Eagle's Creek belt. It was in response to a question about an E&E belt that might escape detection (at least initially)if your were kidnapped/ taken hostage and an opportunity to escape arose.

I managed to cram in to it:

1 20mm button compass.
1 spark-lite.
1 Dermashave razor knife.
1 Dermashave folding saw.
5' 55o cord.
10 Assorted hooks.
40' 30lb Spyderwire fishing line (on a bobbin).
5 Tinder-Quick.
1 ltr ziplock bag.
5 puritabs.
3 Band aids.
5 lifeboat matches.

Your basic limitation per item is that it has to be less than 1" across and no more than 1/4" thick.