Just following up from my comment this morning. CA has a daily non-stop to Zhuhai for about $480 r/t. You can easily get ground transportation into Macau from there. This would give you about 24 hours in Macau.

Something else you might consider:

There is a Turbo Jet Ferry directly from SZX (Shenzhen) to Macau. There are multiple flights a day into SZX on CA, one of which arrives pretty early and another that departs fairly late the next day, giving you a lot more time in Macau even with the added time of the ferry ride. This is slightly cheaper than the flight to Zhuhai.

Keep in mind if crediting to UA on CA flights you want to book a K fare or above to get 100% milage credit, lower fares will earn 50% miles or none depending on the fare.