Just think of the things you use or change all the time . The design is the hard bit .
Maybe something like 2 crossed hex keys ( like skull and crossbones ) with a picture in each segment .
Gears , batteries , finned nitro motor , wheels , shocks and springs , transmitter .
My much more simple idea -
Front - picture of a buggy with aerial ( so you know it is RC , nitro version could show motor through the body )
Back - MY OTHER CAR IS ELECRTIC !!! ... or ... MY OTHER CAR RUNS NITRO !!!
Keep us posted ... I like your " I build them " idea ....If they come in Large Black , I'll buy one !