Bassman:
thumb tacks is the easiest way to go. I do not like golf tees so much.
I found that rolling a tube of green stuff is not so difficult. Let it dry and cut the the two ends with a cutter and, voilà, you get a perfect hat, ready to be decorated.
Like these:
https://discourse.chaos-dwarfs.com/t/8827
Lately I’ve tried thumb tacks filled with green stuff, this is the best compromise as the tack helps you to get the perfect size and shape and green stuff gives the curve and the shape the tack does not have.
I found that making hats is easier than I thought! So no more excuses to not have tall and oversized hats on your dwarfs :hat