How to specify output Coordinate as a parameter for Project tool

In model builder, how do I make the Output Coordinate System of the Project tool select the proper coordinate system based on a UTM zone parameter? We have Lat/Lon (WGS84) coming in as Input Coordinate System but the model will have an output coordinate system that corresponds to the proper UTM zone (WGS84) each time the model is run. We want to have the model select the proper coordinate system based on the UTM zone. But how do we tell the output coordinate system variable of the project tool to get its input from a parameter variable?
Thanks for any help on this. -Lori