Thanks for the tipps, guys!
At first I tried what PandaBerry had suggested. I had an old, baity Skipper from childhood lying around, so I tried to use her scalp. Sadly it still looked bumpy, so I started to experiment with my cloth again.
I'm happy to say that I found a solution that works for me!
I used thick, stretchy fabric and prepared it according to this
video.
But instead of using only one layer I applied two. This made the fabric sturdy, but still elastic enough to make for a great cover! I used the removed hard cap to cut the fabric into the right shape, then I glued it into the hole.
One layer of acrylic paint and one layer of waterproof sealant later, the new material feels very rubbery and is great for rerooting.
Here's a photo of the "finished product":
http://www.imagebanana.com/view/htbpfo8t/IMG_5169.JPGHer ears still need to get glued down and then she's ready to get her new hair! Yay!