Cut the plastic into the shapes shown below. Some notes:
The 3.5" dimension is arbitrary. I feel that it is sufficient, but if you'd like, you can make it longer, to make the T-square more stable.
The 7/8" width of the small piece is also arbitrary, but works well. It is wide enough to prevent it from wondering too much during glue-up and narrow enough to not be huge.
Leave a bit of margin for error on each side of the straight-edge part of the T-square. This is because you will be truing it up so that it is perfectly straight and of uniform width, and you'll need some material to work with.
Once that's done, true up one edge of the straight-edge on the T-square so that it is straight. I tried a couple of techniques to do this, but the only one which I could get to work was optically:
With your eye, look along the length of the edge.
Mark the high spot(s) with a marker.
Clamp the piece to the table and file off some of the high spot(s). Make sure you keep the file square with the T-square.
Repeat steps 1-3 until you can't determine where the high-spots are.
File the other edge of the straightedge until the width of the strip was almost 1" - I went to about 1.015", on average - along the whole length, and then straighten that using the optical technique given above.
Similarly, straighten one edge of the small piece. (actually, I was able to use a factory edge of the Plexiglas, so I didn't have to)
I glued up the T-square using epoxy and the jig shown below. Make sure that the edge of the straightedge and the small piece are in contact with the square.
Make sure you clean off the square (using isopropanol [rubbing alcohol]) once the epoxy gets firm.