[JThiessen]

That's a poor mans way of doing it. Just make certain you use something that isn't going to be a fire hazard or a bug magnet.

Another thing to check is to see if you can find access to the ducts and insulate them as good as you can. Heat loss/cold loss is a big factor in what you are describing. My guess is that the master is the farthest from the AC unit? And its possibly the room that is most exposed to the afternoon sun? You may want to consider adding a secondary AC unit, such as one of the portables, or a window unit to help.