I’ve been eyeballing the Woodpeckers T‑Square forever but could never justify the price for hobby use, so I decided to try making my own.
This model is a simple T‑square head with a 1.5 inch square recess, so you can slide in any 1.5 inch wide piece of wood whatever thickness or length you want. Super flexible.
 

The head itself is 9 inches wide and 2.25 inches tall, with two recessed screw holes on the back for #6 screws to lock the wooden blade in place.
I made two versions:
•     3/8-inch thick
•     5/8-inch thick
 

The only real differences are the size of the lip that hooks onto your workpiece and the recess being 1/8 inch deeper on the thicker version. Functionally, they both work the same pick whichever feels right for your setup.