How did you sep it? You may be able to create the base easily enough. Don't be afraid of sweatshirts, true they don't come out quite as nice as thinner materials, but they should still look great. We did a 500 piece run for a local labor union on Carhartt hoodies in December, of an eagle in flight with a tattered US flag, sim process, 7 colors and 2 flashes, and they were very pleased. Test a little, but go for it for sure.
Steve