Building a Circular Platform using 12" wide boards
Here is a diagram of what you need. Cut each piece exactly the same with a 75 degree angle on the ends. If they are all the same it will go together alright even if you do not make the outside length of each piece be 23.2937.

If you use planed 1x12 pine boards they will be more like 11.25 inches wide.

Dimensions are in inches.