An absolute time clock!
I'm wondering if it might be easiest to put everything together if one first understands why the concept of length contraction became the primary aspect of SR and LET in the first place. To understand that we really don't need to refer to the MMX and neither do we need to know any terminology or relationships which are now part of SR or LET.
To begin, we can describe how the frequency of sound waves are experienced when a person moves with respect to the source of the sound waves. Do you understand the Doppler effect and can you describe that verbally or with math?