Not wanting to pay $200 or more for a flat field panel, I took a look on Amazon. It fits perfectly on my AT115EDT.
I found something called "Hercules flat field panel".
It came from China, seems reasonably well built. The site where the DC wire goes into the panel itself looks like it needs to be watched carefully, but I've read this about more premium panels as well so I can't hold it against it.
Used it tonight with great results. APT flats guide had no problems creating a good set of flats for my first astrophoto tonight.
If you're looking for a reasonably priced panel, I suggest looking into this.