A bit weird looking perhaps, but it does the job! Add these levers to the bottom of your controller using glue or double sided tape and you can reach both back buttons with one hand.

I do this because i play with a "hybrid" setup using both a controller and a mouse, and hence, I need two buttons for my left hand to jump/crouch in fps games.

I printed mine out of PLA so that i could easily heat them with a lighter and reshape them as needed, but i guess other filaments work to, some might even help reduce flex.

I used parts of a paper clip as pins to assemble the levers but anything round with roughly 1,5-2mm in diameter will work.

short video of it working:

paddle mod (on printables)