I've experimented using a hammer and some of my existing bags. Haven't bought a hawk yet, and I'm not going to till I'm sure I can carry it comfortably on the back of my bag. Simply don't want one if I have to carry it anywhere else.
Here's what my fiddling has taught me:
Bags made from 3-4 oz leather are too soft all by their lonesome. I'd want the sheath made out of 8 oz leather or so. Might even consider sewing a stiffener panel of 8 oz into the back of a bag or making the back of the bag along with the sheath entirely out of 8 oz.
I keep popping back to the 8 oz because that's the heaviest weight I've got on hand. Another might work as well or better, but I'd be reluctant to go lighter. I've got a bag I made out of 8 oz, and though it was a rassel to make it, the resulting stiffer bag just seems to hold the weight of a hawk..... excuse me :redface: .... a hammer...better than softer bags.
The problem I'm trying to get around is that the weight of the hammer or whatever on the back of a soft bag really distorts it and causes problems for me when trying to use the bag as intended. Other designs than what I'm using (the fowler bag from the pattern here on the site and variations) will almost certainly be different.