Yea man, there's a couple of options I reckon would work. The first one is Bondaweb - it's basically like a thin film of fabric that you can cut to size, place between the number and the shirt, then iron over it, and it will stick:
Another one is fabric glue. I can't remember the name of it, but it comes in a little yellow and blue tube, and is basically liquid latex, and can just be used like glue.
Both of them should be avaliable from most fabric shops.
Having a girlfriend that does Art + Textiles comes in handy now and again