The 2x4 Contest Rules stated that we were allowed to use only an 8-foot 2x4 and fasteners. I ended up using most of the 2x4, two 1/2" bolts 8" long, four washers, and two nylon lock nuts.
I used wooden spacers between the wheels and cut the bolts so that only a small portion of the threads extended past the wheel for the lock nut to fit. This allows the wheels to spin on the smooth part of the bolt which is lubricated with paste wax.
Download my Skateboard Template if you would like to make one of these yourself, although I highly recommend using real skateboard trucks and wheels if you are not restricted by any contest rules. The wooden wheels work ok on a smooth surface, but there is no steering and they probably would not last very long. Use at your own risk!
My Skateboard ended up receiving the Most Functional Award and won me a gift card to Peachtree Woodworking Supply! Awesome! Even though the skateboard is functional, I plan to keep it in my shop for display only. I don't need any broken bones!