If you are going to make a new bracket, I suggest you do not make the "L" bracket. I heard that these broke easily and were replaced by a "U" bracket.
I have a "U" bracket that came with my Yaesu g 5400 rotor.
You can see a picture of this in the Yaesu 5500 rotor manual that you can download from the yaesu website.
I have measured mine and have these measurements
length =4 3/4 inches width = 4 1/4 inches
Vertical ends are 2 3/8 inches high
Holes are 1/2 and 3/4 inches in from the ends of the plate. (you will have to mark these with your rotor to be sure).
Holes are 3/4 inch in diameter. There is also rubber inserts/spacers in the mounting holes.
Slot in the middle of the bracket is 3 inches x 3/4 inches centered in the middle of the plate.
It may be easier to just order one from your local amateur radio store from Yaesu.
My manuals for the 5400 and 5500 do not give part numbers.
Hope this helps
Perry WB8OTH