If you have decided that phonology is all about the mind and phonetics is all about the body, then all you need is a drawing of a person, dichotomized into the mind versus the body, and you can label the parts. Good luck with that.
However, I disagree with your initial premise (though I do recognize that that is the standard undergrad edu sound bite). In fact, phonology is about mental manipulation of sound qua symbol, and phonetics is about how those symbols are realized / perceived. Before you get to actual neural impulses and movement of air and muscles, there is a lot of pre-physical planning. Some of this is quasi-scalar, so for example "voicing" during obstruent stops is not uniform across languages (and I mean even excluding situations like English where [g] is for some people voiceless unaspirated), voicing can be strong (higher amplitude, less prone to decay) vs. weak. The path of coarticulation from vowel to consonant is not uniform across languages (e.g. Marshallese vs. English vs. Turkish).
The real question is, why would you want to explain this difference? The simplest solution is to say that it's very complicated and controversial, but here are a few approaches (and then explain some of them). Or, if there is a real point that motivates you (such as that phonology does not deal in continuous functions or numeric values and that is has rules / operations of a particular type), then define the difference that way. The graphic should be trivial and really unnecessary, once the concept is clear.