Just a tiny bit of work on the utility arm mystery project tonight.
I sanded smooth the styrene front tip piece. The dark areas that look like gaps are actually filled with glue, so I don't think those areas will need to be filled.
I then tried it on the droid.
It doesn't quite fit like a glove, but I think the fit is good enough for its purposes.
I need to do some slight trimming of the overhang on the front-most surface, and I have one more minor piece to glue on. Then I'll plan to spray some primer to see if I have any areas in dire need of filling.