According to this sheet it's pin 28. (click for bigger version) I can still see the contacts of the pin so I might be able to solder directly a wire to it.
Yesterday I cannibalized a 6581R4 chip out of a C64c but the last pin broke off. Is that pin used by MidiboxSID? Or isn't it a problem and can I just use the SID?
I like your design but I agree with Raphael that buttons under the LCD are more ergonomic. I would add some LED's too so you can see a parameter is configured by just looking at a led (and ofcourse, for the coolness of it 8) :P).
I'm planning to make a SID myself too. But it's pretty hard to get hold of 4 SID's. And it's a 'waste' to wreck 4 c64's for 1 SID. I'm getting one SID out of a c64 and I'll use that case too. But where to get the other 3? Is there like an online shop? Or do I have to get them out 4 other c64's ;)?