I made my spokes out of treated 2x4's but you can make them out of any good, straight 2x4 material. I chose treated lumber to help prevent warping once this whole thing gets out in the weather.
I used the miter on my table saw but that's probably not the safest way to go. If you do, be really careful.
Now drill two bolt holes in each spoke to fit the holes in your hub.
Next we countersink the holes to accommodate the square portion of the bolt that will stick through the hub.
Now that we know exactly where the spokes will mount you need to cut them to length. I made them so that they sat about a 1/4" inside the outer circumference of the wheel.
The final step in making the spokes is to dado a grove for the bottom boards to fit in. I made mine a 1/4" deep.