Awesome!
FWIW, I made a few by hand, despite doing all the prep work to CNC them. Long story but the CNC I had access to didn't have spindle speed control and at 12000 RPM it would melt the plastic instead of cut it. Pretty messy and smelly.
I used ABS with a haircell texture on one side. I don't remember the exact thickness but I still have some and can measure if needed.
There is more variation between backplates than you'd expect. I did three plates for various flys. They were all different, none matched my other flys either.
The process I went through was scanning the back of the guitar on a flatbed scanner, using a specific DPI setting of 300.
Then, I imported that image into 3D CAD, I could scale the image (if necessary) by comparing the diameter of the bushings in the image to measurements on real parts.
I used the image as an underlay to create the outline and hole centers. Then I exported that as .dxf to create a CAM file and get g-code to the CNC.
If you included the machine time, this took 1-2 hours, and then it would be 10 min for each plate after that. So $56 isn't too bad all things considered.
After trying for a while to get the CNC to do what I wanted, I opted to just cut them out on a scroll saw and drill everything carefully by hand. Clean up with a file and sandpaper.
Terrapin probably have a CMM and can export hole centers and profile right to the CNC, so much quicker but still probably 1 or 2 hours.
There are some cool infused graphic options for Kydex sheet that could be used too.
https://www.ebay.com/sch/i.html?_odkw=kydex+sheet&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H1.Xkydex+sheet+graphics+infused.TRS1&_nkw=kydex+sheet+graphics+infused&_sacat=0