Modelled completely from scratch using photo reference and a combination of Fusion 360 and TinkerCAD for minor alterations. Actually using it as the basis of a replaceable backplate mod which swaps out the stock backplate for one that includes a grip to make the console more comfortable for larger hands. While I work on that I thought someone might benefit from a replacement backplate - now you can print one in any colour you like or, using this as the base for your remix, modify it for different a grip, texture or whatever you fancy!
Test printed and replaces the stock backplate perfectly after fairly minimal clean-up.
I highly recommend a print resolution of 0.1 and a slower print speed to ensure all the small parts print correctly, I also used a .2 nozzle but you should be fine with a .4 nozzle - just may need to dremel the holes to clean them up. I would also suggest printing in PETG for the the aforementioned reasons. While PLA will work, the smaller parts are very fragile when printed in PLA and the last thing you want is broken bits rattling around inside your controller.
You'll need a tri-wing screwdriver to open up the JoyCon, look up a joycon teardown video on YouTube to make sure you don't damage anything!
AmazonBasics used for testing PETG recommended
Disassembly and reference.
Removed the backplate from one of my JoyCons (4 tri-wing screws to remove the plate and 3 internal phillips screws to remove the rail circuit board). Took a well lit, high resolution photo of the inside of the plate and imported it into Fusion 360.
Basic model in Fusion 360
Using the photo as reference, it was a simple matter of scaling the reference correctly (measurements made with digital calipers) and then sketching over the photo for the basic shape and layout. Used the fillet tool on the curved edge to recreate smooth edge.
Prototyping and quick alterations in TinkerCAD
I prototyped the Fusion 360 model with my printer, mostly fine but it highlighted some areas that needed reinforcement or slight scaling/position issues. Some of these were corrected in Fusion 360, others were quicker to bodge in TinkerCAD (ended up altering the design for the trigger hood and completely remodelling it in TinkerCAD).