Regarding the screws on top of the C11, I had a similar problem with my C11 EdgeHD as I wanted to mount a bar. The screws on the rear towards the mirror were long enough (there are 6 of them with the same thread, but of different length, so try removing all of them and choose the longest, the others are fine for the finder mount). The ones on the front towards the corrector plate were too short and had a different thread compared to the ones on the rear; I bought some M5 screws, and they fit not perfectly but good enough (I had to force them a little bit). The original screws go perfectly inside an M5 threaded hole, so I guess they are really M5, but the screws I bought were slightly larger by a couple of hundred microns. Others confirmed me that Celestron is using metric screws. By the way, the screws I bought were really too long, so I had to cut them to the needed length. Be careful if you have to cut them because the thread may be slightly damaged where you cut; before cutting, screw a nut inside, and after cutting unscrew and screw it again several time to restore a good thread.
Ciao, Claudio.
