Use a Nintendo DS as a wireless Switch controller, using a Raspberry Pi Pico W as a receiver.
You can tap the center of the screen where it says which joystick is selected for L3/R3. You can also press volume-down on a DSi for the opposite L3/R3 button. The power button also works as the home button on a DSi.
In addition to the on-screen buttons, you can press select or volume-up and one of the following buttons for extra options:
A: Trackpad mode
B: Hide buttons
X: Send home button when lid closed
Y: Show/hide clock
L/R: Next/previous background on top screen
UP: Send A when stylus is down (only in trackpad mode)
DOWN: Toggle bottom screen backlight
LEFT: Toggle 4-way D-Pad
Start: Home button
More custom backgrounds can be added in the nitrofiles folder, though it requires changing NUMBGs and recompiling.
Flash dscon.uf2 onto your Pi Pico W and plug it into your switch. Then run dscon.nds on your DS and it'll automatically connect.
DS code uses BlocksDS. Most of the Pico code is from https://github.com/knflrpn/SwiCC_RP2040 .