SRAM MTB cranks have a bunch of ribs on the back side of the crank arm. That means you have to get a new crank or use pedal or spider based power meter. Pedal based power meters aren’t the best match for MTB because people tend to strike their pedals on rocks and tree stumps.
Sigeyi and P2Max make relatively inexpensive spider power meters that are reliable. The thread below has lots of info.