Wanting to formulate some FM waveforms by myself, and have a greater visual input and instant audio changes, I found this here: https://bgc.github.io/waveformgenerator/#/
Yeah, I know we have a bunch of waveform generators in Renoise, I found 'em all. This one just provides a bunch of ideas and provides instant, visual feedback on what is going on. Outputs single cycle waveforms and you can drag 'n drop 'em into Renoise. Quick and easy.
I wanted that Taco Bell FM piano sound. Made it in like 10 seconds. Nice!