Well i took pictures and put sizes on it, but i seem to have forgotten to put a size on the small retainer that sits on the inside. Basically the hole should be drilled so the front of the bend sits flush with the main 1/4" plate, its not designed to sit over the front but flush or a little inwards, its basically a spacer for the seat bar to hinge on, the main block should be able to *float* a bit to get over the bumps on the seat bar.
If there is anything thats un-clear just let me know, you will see a shot of how its all assembled with a welding rod showing where the seat bar should sit in this arrangement.
http://users.tpg.com.au/tazza_/seat%20bar/
Let me know how you go, the main part is made of 1/4" plate and the angles seem to be 45 degrees. Let me know how you go.
Remember, the angles don't need to be spot on, it has a bit of give in the whole setup to compensate for wear and mis-alignment.