I prefer thin plywood for shaped brackets, its a measurable thickness. Once the plywood shape is finalised plus the hole positions then stacks of known thickness washers can be used to get the correct thickness at the hole positions. On my P3430 Brembo adapters there are 4 holes with the 2 pairs at different heights.
This is my adaptor for the disc to hub, 5 holes to 6 holes at different heights and this was prepared in the same manner, no CNC machinery, DRO is as far as I have gone on my 1942 Mill.
Glass 5 micron scales on Mill
DRO computer
Display, 4 scales combined into 3 axis on the Mill, this is the lathe DRO display 2 axis
Mill DRO Display
Hole position programming