Not sure when you are flying ... but C cabin does not appear to be full for the next 2-3 weeks (unless I am reading this wrongly).

UA 863
F8 C9 D8 Y9 B9 M9 H9 Q9 V9 W9 S1 T0 K0 L0 G0 A7 Z6 E0 U0

UA 870
F9 C9 D0 Y9 B9 M9 H9 Q9 V9 W0 S0 T0 K0 L0 G0 A7 Z0 E0 U0