I have a DP helmet and I like it a lot. It runs a little small, but since I'm small, it works. However, it was a lot of work to make it useable, so keep that in mind.
Now, I'm not sure exactly what your question is. You should mount the visor behind the cutout, so the visor should be bigger than the cutout. Put your epoxy on (thick enough that it will fill into the gaps and touch your visor's extra width evenly) around the cutout and put your visor into place. Be VERY careful that you do NOT get epoxy oozing out onto the visor itself though!
I agree with Matt--the Dented Helmet is THE resource for building a Fett. Buy a reference CD from them and check out the website. We can give you some of our advice, but honestly, we pretty much went to TDH to build ours, so we will mostly be repeating what you see there.