Mavlink to FrSky S.Port Passthru Converter for LRS or PX4 Pro

Daniel,
with your setup you do not need a teensy, all you need is an inverting cable

The wiring is:

FC → inverting cable → X8R s.port

take a look at

http://ardupilot.org/copter/docs/common-frsky-telemetry.html#common-frsky-equipment
http://ardupilot.org/copter/docs/common-frsky-telemetry.html#diy-cable-for-x-receivers