In case anyone else is interested, I decided to try Tee Shirt Express on Thepprasit Soi 3 to get some custom shirts printed, it's not far from my house. I took my designs on a USB and the guy let me try on an unprinted shirt to check the size is right (3XL as it happens), and then said to come back in three days. I ordered three shirts with different designs and I'm pleased with the results, it's almost impossible to find tee shirts with nice, original designs in Thailand, it's all beer makes, or Same Same but, or Good Guys Go to Heaven, or similar nonsense. I just picked them up and I'm well pleased, I think they were 410 Baht each. Here's one example: