Brad:
All I can say is that the dimensions for the slots are 1" for the standard combination ruler. And 2" for a framing square.
If you're unsure if your square will fit the slider is to buy a setup all assembled. That is I'll provide you with the combination square ruler, and a framing square, for the cost of both these tools added to the total.
Both were bought from home depot tool section and are standard tools available.
The framing square in the photo is an aluminum one called a "fat boy" as I remember it. I'll go to home depot and get the part numbers for the ones I got that I used to make sure we have the correct sizes, including prices.
The metal is aluminum.
If you're not happy with one I'll gladly return you money less shipping charges each way.
Paypal preferred, if it's easy for you.... whatever works is ok..

Jim Rogers