comments:
The ultimate 2 controller has back paddles: https://www.amazon.com/dp/B0GTYV7BG5 Spending 60 on one of these will be much, much less work than what you are looking to do. Adding back paddles to a controller that does not have them in a way that isn't terrible will involve a ton of design work and testing, on top of wiring a new button to the old buttons switch and making a housing/mount for that and getting your back paddles to push it correctly. All of that will suck. It's not impossible, but it will suck. Of L4/R4 are not clicky switches like they are in the ultimate 2, it will suck even more as a clicky switch will easily have a wire soldered on, but a soft pad like the L1/L2 bumpers will be very difficult to work with, and will fail over time unless there are nice clearly labeled testpoints elsewhere for L4/R4 that you can use instead. Just buy an ultimate 2. It's a stellar controller. I have an ultimate 2C which I liked until I picked up an ultimate 2 and dropped it entirely. The ultimate 2 also adds 2.4ghz which is a dream for low latency wireless.