Fair enough
The full answer is to use the quatosTool program (which can generate mixes for PID well as Quatos). You'd define your custom frame in an XML file and use the program (comes with QGC) to generate a mix. Basically you'd need to do something like this:
http://autoquad.org/wiki/wiki/quatos-ad ... tribution/ but for PID controller you don't need to get as detailed about the masses.
I see custom frame layouts aren't documented in the wiki, but there are examples and . Basically you just need to specify your motor X/Y coordinates in the geometry section.
Also I'd wait a few days because a new/fixed quatosTool (and QGC update) are coming up.
Having said all that, and having tried a bunch of frame configs for PID mixes with quatosTool, I bet it will generate the same mix as for a quad X.
-Max