I machined two pieces at a time and used two more pieces next to them to make sure my clamp plate stayed flat. The holes are 3/4" apart which was needed to work with the material below.
 

With the ends of the larger angle trimmed, I'm ready to start assembly. Below the larger piece at the top, I cut the 1" square angle into 3" lengths. Next to that are some 10-32 button head cap screws along with some locking nuts.

I'm using two different sizes of flat washers, #10's and 1/4". The reason for that is because the slots in the angle are just larger than my #10 washers so I'll be using them stacked upon another.

Last but not least, is the .008" shims that will be used, two per wheel.
 

I did my assembly using my bench vise as it acted like a third hand. You can also see how I'm using two pieces of the smaller angle, creating a U shape. One angle is for the .008" shims, and the other will be used to hold a Reed Switch. If you look close you can see two shims are being used, one on each side of the angle.
 

A view from the other side. The shims click when the wheel turns every 1 1/2" which should work out nicely. If you're old enough to remember, think playing cards in your bicycle spokes and you'll get the idea here.
 

The next item to attach to the slotted angle will be a Reed Switch. I'm sure most of you have seen this type of switch before because they are used in burglar alarms. Inside the piece with the three screws is the switch. The piece without screws is the magnet. When the switch is next to the magnet, it moves the contact inside and either 'opens' or 'closes' the circuit. One wire goes to the COM or common terminal and the other wire goes to the N.C. (normally closed) or the N.O. (normally open) terminal, depending on your needs. However, I won't be using the magnets that came with these switches, I have something else in mind.