As I said before, I'm going to use a steel axle that will go through all the wheels but it will be stationery. Here I'm cutting through the 5/8" diameter axle that will span from one side of the frame to the other. The silver slot is where I stopped the saw to take a picture.
 

Here is what it looks like mounted to the frame. Also notice the two 10-32 socket head cap screws (black) at each end now. These are to keep the axle from turning, but lets take a closer look on how I got here.
 

I put one hole at each end of the axle for the 10-32 screws to go into. However, the holes don't go all the way through and are only 1/8" deep. I used a 1/4" drill for this so I have some adjustability for my #10 screws. Now the first hole is the easy one, but I needed both holes in alignment so everything lines up properly.
 

The first hole I drilled is on the left with the # 10 screw fastened down tight in the block. Next I used the milling machine table as a surface plate and tightened the other block down, making the bottom of both blocks parallel.
 

And there we go, both ends of the axle now have holes that are in alignment with each other. The four mounting holes in the aluminum block are slightly oversize to aid in adjustability for the four #10 wood screws.
 

Remember all those pins I put into those 5" diameter wood pieces? Well after playing with different size feeler gages, I found that using two .008" gages worked out pretty good. Once I knew that, I bought a few feet of .008" thick shim stock. The material is stainless steel and is in the full hard condition.