For medium creatures, what you can have instead is a conical hat (I'm sure everyone will agree that the correct shape for a wizard's hat is a cone).
Vcone = A/3 h
VHollowCone = A/3 h - A/3 (h-x) , where x is the vertical thickness, since the volume of the frustum you get should be equivalent to that of a hollow cone.
As calculated earlier, we have a 8.163 ft3 volume of adamantine to work with. We want the radius at 6 feet to be at least 1 foot (so you can stand upright in your unshrunk hat, no problems. Furthermore, let's have the area of the base be equal to 25 square feet (equivalent to a 5x5 square), so for future reference R = 2.8 ft.
8.163 ft * 3 /25 = x
x = 11.75 inches.
The angle of the cone depends upon our height. Let's make it, oh, 8 feet. That makes it nice and pointy.
T = x*r/h
11.75 inches *2.8/8 = 4 inches
This has three advantages.
First: the idea of a wizard's pointy hat expanding to protect him is something that wouldn't be out of place in the classic portrayal of wizards. And wearing a pointy hat isn't exactly suspicious for a wizard. Since shrink item can turn it into a "clothlike composition", it's perfect.
Second: It works for medium creatures
Third: A leap attacking fighter is now jumping onto an eight-foot-long spike.