I made the gauges as I wanted to use as much of the original bmw cluster as possible. The gauge bodies are made from some 5 inch aluminium pipe turned down to the right size. I mounted the original speedo and rev counter motors on some acrylic sheet and trimmed it to size to fit inside the body.
The dial faces are vinyl stuck onto the acrylic and then some clear arcylic was used to seal the gauges along with the chrome trim ring.
I just used a lathe to machine the aluminium pipe to the shape and size that I needed. I cut a slot into the outside of the pipe to enable the trim ring tabs to sit against it. The gauges have an aluminium back plate that the leds are mounted in that provide the backlight.