Couture: Bumpy

I wanted to screen-print onto the sleeves of a shirt.  It turns out that’s not trivial:

When the fabric is bumpy (the seam down there), the screen doesn’t make contact properly with the fabric, which makes the print crappy.

So I need something soft-ish under the shirt to ensure proper contact with the screen.  Something rubbery, perhaps…  I’m not quite sure what, but I’ll visit a hardware store tomorrow and see if I find something.