My elevators on the horizontal stabilizer (my black-gloved hand is touching them) need mass balance to minimize flutter at high speed.
My rudder, also in 1st photo, has the mass balance built into it. There is not enough room to do that for the elevators.
Where to put the mass balance?
The oval hole at the bottom of the vertical fin is an access/inspection point. It goes there.
Note: when the mass balance is 'far away' from the control surface, the control system linking the two should have as little slop (backlash, hysteresis) as possible.
https://en.wikipedia.org/wiki/Backlash_(engineering)
https://en.wikipedia.org/wiki/Hysteresis
The counterweight is lead shot in thickened epoxy. (I won't melt and cast molten lead!)
The density of lead shot?
https://en.wikipedia.org/wiki/Sphere_packing
Read alt-text for more info.
Info on mass balancing, and a view of my rudder with its integral chunk of lead:
https://universeodon.com/@KrajciTom/114484536950179218