Step 2: Making the base, and mounting the wheels
To make a nice, sturdy base I cut the 2x4's at angles and put them together to make two triangles. You can make them whatever size you need, depending on the size of your panels. I then tied them together with a couple of 2x4's at the base, and a couple up top. This made a nice, sturdy base to mount the wheels to.
I cut a couple of small pieces of angle iron with a hacksaw, found the mid point on the cross members, and attached them exterior woodscrews. I put the wheels through the holes, and spun them with satisfaction.
Here is a picture of the top wheel being mounted.