We thought about this problem for a long time before we came up with a good way to do it. A turret needs two things to work well: a good support base, and a means of transmitting power to it (for turning). Unfortunately, turntables and big sprockets are expensive and need to be ordered. We also don't have access to any lathe big enough to make round parts that big....
Eventually we came up with this solution: a v-belt pulley for an evaporative cooler with the hub cut out and supported by bearings on the outside. This set up has the neat feature of having only one round part; the power and support are both provided by the pulley. The best part is that you can get one at the local hardware store (in AZ) and it only costs about $30.
Note: the bearing in the picture is going to change to a normal ball bearing... we didn't have one the right size at the time of building it, so we just faked it with what we had on hand.