This is what I use on my desktop slippi to get my LBX to read properly.
[Profile]
Device = DInput/0/Arduino Leonardo
Buttons/A = `Button 0`
Buttons/B = `Button 1`
Buttons/X = `Button 2`
Buttons/Y = `Button 3`
Buttons/Z = `Button 4`
Buttons/Start = `Button 8`
Main Stick/Up = `Axis Y-`
Main Stick/Down = `Axis Y+`
Main Stick/Left = `Axis X-`
Main Stick/Right = `Axis X+`
C-Stick/Up = `Axis Yr-`
C-Stick/Down = `Axis Yr+`
C-Stick/Left = `Axis Xr-`
C-Stick/Right = `Axis Xr+`
Triggers/Threshold = 0.0000000000000000
Triggers/L = `Button 7`
Triggers/R = `Button 6`
Triggers/L-Analog = `Button 7`
Triggers/R-Analog = `Slider 0-+`
D-Pad/Up = `Hat 0 N`
D-Pad/Down = `Hat 0 S`
D-Pad/Left = `Hat 0 W`
D-Pad/Right = `Hat 0 E`