I haven’t done this for years, but I used to design custom t-shirts for myself. I still wear them, and people are sometimes surprised to find out I made them. I’ve had people ask about where I got them or mention I should sell them. So that’s what I’ve started doing.
The conversion has been a bit tricky. The shirts I custom-designed were made on vistaprint.com. It’s a good site for designing for myself; however, they only offered three colors when I was making them: white, light gray, and black. White got kinda dinghy after a bit, and stuff showed up on the black shirts, so I ended up with several gray shirts. I stopped making them after I got tired of filling my closet with light gray shirts.
Anyway, I’ve managed to convert a couple designs into potentially broader base; I also created a completely new design that I’ve never worn. If you get a chance, check out the selection of shirts (and other items) on my store: https://www.redbubble.com/people/EdwardAntonym. There are currently three designs, but I’m planning to add more as I get the chance.