Still looking into making some of these carb plates, just need to find a good local source for steel.
I have a 1' long piece of 1/2 thick x 6" wide aluminum angle stock. I think that would work well and it would be easy to put in a vise and use. I'm just not sure how hard it would be to mill up.
Metal depot or Mcmaster-Carr, few bucks for some aluminum, literally 5 minutes with a drill and you are done.
The 1/2 inch thick aluminum will work, no need for a mill. Lap with some wet dry sandpaper on a table saw bed or on a piece of window glass until smooth.
Lay out the holes, drill, no need to tap the mounting holes, just drill and use some #10 screws to clamp the carb to the plate. Remember, you are bending pot metal not titanium. Use some washers for the #10 screws to raise the low areas to aid in bending the carb body.