I had the same job done on my C240, with the same curved compartment problem. Had the pocket cut at a local store but I must say I'm not all that happy with how it looks. Its acceptable, but not great. I would love to see pictures of your install.

Regarding the FM Modulator, I have found that the cassette adapter from Radio Shack (I tried others, e.g., from Sony, and they did not work as well) is capable of providing way superior sound quality than FM modulators (I've tried Clarion and JVC FM modulators). If you can live without your cassette player, you can use the cassette adapter for the much improved sound. In my head unit, if you route the cassette adapter cable exactly through the top center you will be able to NOT get the constant beep from the unit thinking the cassette door is open. You may need to carefully force close the cassette door. You can then route the cable of the adapter through the back of the unit and hook with the Empeg. See attached picture of how it works on mine.