If the UA website isn't giving you any options with a longer connection try google.com/flights to find the flights. I'm seeing options through IAD that are 2 hours 10 min or so which while tight is better.

If that doesn't work try searching for it as a multiple destination trip. Depending on the fare rules it should price the same coming in from google.com/flights and it allow you to pick which IADMCO to get a longer layover.