I just got done having that same problem. Needed to make a t-square. Its not any easy thing finding a tool that makes an accurate 90 degree angle. You need to use a good square or you need to make a pattern and cut it out of wood.
Not sure if I can explain this well but I will try. Draw a line 3 units long (or multiples such as 6,9,12,etc). Label one end "A". Label the other end "B". Place a compass on point "A" and scribe an arc 4 units long (or multiples such as 8,12,16,etc). Now place the compass on point "B" and scribe an arc 5 units long (or multiples such as 10,15,20,etc). Draw lines from the intersection of the arcs to the points "A" and "B". You now have a 90 degree angle.
You can make it any size you want by using multiples of 3,4 and 5. A triangle with sides of 3 and 4 (or multiples) will always have a hypotenuse or 5 (or multiples).
You can draw this on plywood and then cut it out and then plane or sand the edges smooth. And yes, you can use this pattern with a router using a pattern bit. Or you can draw it out and then use a straightedge to follow it with a router.
I'm sure others will tell other methods.
Last edited by sofasurfer; 02-21-2010 at 05:47 PM.