... your build almost mirrors mine (in terms of receiver parts) - Seekin Precision mag release & bolt catch; CMT lower; Magpul furniture;BAD-ASS(FDE); CMC or Geissele flat trigger (trying to find someone who has a CMC flat trigger to try it out first before I make any decisions). Good to know about the ease of assembly for CMT's lower.

Curious - is there anything wrong with the traditional metal dust covers? Or did you just want to go with something different? Also is the dust cover dimensions for the UPUR-4 any different from other uppers? ... it looks a tad bit longer.