It is really cool that we have "female t-shirt" options now at DrupalCons. Though, with my size/shape, I prefer men's t-shirts. And I have quite a few of those. What I didn't have is a Drupal t-skirt. Until now.
I posted up instructions on how to make a Drupal t-skirt. Though I should get a better photo.
Basically, you measure the widest part of your hips. Divide that by X number of t-shirts. Say 40 inches at hope, 5 shirts, then each should be 8 inches. With 1/2 inch each side for sewn edge. That is how wide to cut the top of each t-shirt panel. Then just cut a diagonal to the bottom widest edge of t-shirt. I used a waist band from a tighter fitting t-shirt. I assembled and sewed it all in a sort of slap-dash way, showing the seams and stitching.