Another option: Modify your cold air return system. A cheap and efficient way to help cool the house. We have a similar set up on a Tri-level (garden, main, upstairs). We have two cold air returns one on the main level and one upstairs. The cold air returns are floor level, are 32 inches wide and had a fixed grate. If your system utilizes the wall itself, you can install a louvered grate at floor level and cut out a Hot Air return at ceiling level directly above the cool air return. Install a louvered grate at ceiling level also. In the winter, close the louvers on the Hot Air return and have the Cool Air open. In the summer when you are running the AC, close the floor level Cool Air return and open the Hot Air return.