One of my biggest problems in my workshop is a 4 x 8 sheet of plywood. With this T-square I’m able to cut it down and make manageable pieces ready to cut to size on the table saw.
Supply material,
12 x 13 piece of 3/16” hardboard
38 x 7 piece of 1/2” flat plywood
2 knob bolts 1/4 - 20
2 bolts 1/4 - 20 x 1/2”

Step 1: 3/16” Hardboard Square

1st step is to make a hardboard (mine is 12 x 13) a bit bigger than your circular saw base. Place saw squarely on base to mark the holes for the T-nuts that will attach the saw base to the hardboard.

Step 2: Add T-nuts

Counter sink 1/16” deep a 7/8” hole so the t-nuts will be below the surface so they won’t scratch the plywood you cut. Now drill the holes through with a 5/16” drill to accept a 1/4 x 20 t-nut.
After inserting the t-nuts I had to grind them down a bit so they didn’t exceed the thickness of my saw base.

Step 3: Resess T-Square Arm

Resess the t square arm to accept the hard board that will hold your saw base. Your T square arm could be as long as nessary, but mine is 38” and I’m able to make a 36” cut with it.
I resessed mine about 5 1/2” , 3/16” deep. That way it has a good glue surface and goes right up to my saw base.

Step 4: Routing T Slot Arm

I routed 1/4” slots to accept the 1/4” Knob bolts I use to adjust the width of the cuts I want to make.
Also on the bottom of the arm I routed another 1/4” slot 1/4” deep to accept a 1/4” key way to keep the T square on the arm.

Step 5: Making the “T”

I made my T 17 x 3, and it works nicely. I have 2 nut inserts and the key way mounted on this pretty simple.
To mount the Key Way which should slide nicely in the slot but not to loose. I placed wax paper over the slot put key in the slot glue on top only of the key, then attached the T piece and made sure it was all square.

Step 6: Finishing Up

Finishing up you attach the saw to the base squarely, with the blade up, then plunge the blade through the hardboard.
I also glued a piece of wood to the arm on top just to stiffen it a bit. I added a sticky back ruler to the arm to finish off.
Hope this will save you some time and make your time more productive.