Salvage a genuine DSM2/DSMX module from a cheap Spektrum DX4e or DX5e transmitter, and convert it to a JR-style module for Taranis, 9XR and other transmitters.
These put out between 60mW and 100mW, depending on your luck. Spektrum individually tests and marks the output power (in dBm) on the back of these modules, so you may want to open a few up and select the one with the highest output.
Besides the spectrum module, you'll need about $5 of cable, 3.3V regulator and antenna.
You may also want to to look at Dan Myers' similar project at http://www.thingiverse.com/thing:293608
I built this thing after looking at his (thanks, Dan, for the motivation!) because I wanted these differences:
- Antenna is mounted higher
- Series voltage regulator instead of switching regulator to prevent EMI (switchers emit RF that can interfere with sound and signals)
- Cleaner internal board mounting