Having used all of the suitable field caps from the Perry's plastic Prussian infantry to convert my ACW infantry into Mississippi Marine Brigade types, I thought I'd have a go at sculpting some of my own.
After a little experimentation I think I've come up with something that'll work.
The Perry ACW figures come with whole heads that are bald but for a rim of hair. You can then glue on separate caps or hats. I've actually painted one or two in the past as hatless chaps with male-pattern baldness! This has the advantage that the head provides a nice large surface on which to sculpt alternative headgear.
My developing technique is to put a blob of GreenStuff on top of the head and shape it into a low cylindrical shape. I'm being less than scrupulous about cleaning up the sprue mark on the top of the head as it adds some roughness that helps the Green Stuff to stick. Sorry, pictures that follow are the final result, not step by step.
The crown of the hat is then another ball of Green Stuff that I've previously flattened into a disc. Press on and push into shape.
Finally, a small piece of Green Stuff forms the peak. I've found the best way to apply this is to shape it roughly before applying it and pressing it onto the cap with a sculpting tool. I then squeeze it into shape with a pair of fine pointed tweezers.
I need to paint up these samples to see what I think of the finished items. If I'm happy they'll save me buying a whole box of Atlantic Prussians just for the caps!