Iron ons are pretty tacky. The best and cheapest way is to create a simple design that only uses one or two colors and screen print it. There are many ways to do that, but the easiest method would be doing something like this
.
Protip: Embroidery hoops work well if ya can't get a hold of a cheap frame, and you do not need to cut out the design. One could trace the design lightly on the screen as well.
For example, I started making this for a personal t-shirt based on one of the pumkins at hhn 20 (never got around to finishing it though). Something like this on the back (perhaps with some glow in the dark fabric paint) would look neat.