Marc replied that Ford's head is a long oval shape so he didn't need to add any thickness behind the sweatband to get these results.If you add thickness on the front and on the back the hat will stretch on the head giving a reverse taper. You could tell me that in this case we couldn't see the back side straight as it is in every scene of raiders... well if you push the back dent far back you will always get the same look... and as a matter of fact in the "basket game" scenes we see the back dent is pushed so far back that someone on COW wondered how could be this possible without getting a taper. The fact is that pushing it far back is just a way to hide that the hat was stretched.
Stretching it on the front and back like you can see I did in the pictures I sent you causes reverse taper on the sides.
With a thickness under the sweatband on the front would stretch the hat getting:
1) reverse taper
2) the same brim loops seen in some fighting scenes
3) as a consequence you'll have to push back the center dent like we see in all the action sequences where Ford needed that the hat didn't fly off his head.
4) this trick also hide a tapering that surely happened under the hot tunisian sun
I hope I was clear enough. Have a look at the pictures
For the regular oval heads I might suggest you a bent tissue on the front behind the sweatband, It works quite well.
Now my pictures
FRONT SIDE... normal / stretched
Tell me what you think