Controller
Servos supposed: https://www.servocity.com/hs-785hb-servo It rotates 4 full turns, so it has to be constrained electronically. The positioning precision will degrade of coarse but for purposes considered should be still enough, I believe. There is a lot of servos of same size/case, may be more suitable/cheaper can be found, I just didn't have enough time to search more diligently, didn't know you would extend competition. Also, It's torque, 13 kg/cm, might be excessive, smaller servos would be cheaper. By your description the torque needed is about 2 kg/cm. It seems like a good idea if you try to measure it somehow, a mistake at this stage would take later much money and time to correct. I can re design for any other servos, no problem. An extension down at right side is intended to act as a lever pressing down the left side as the belt on that side goes horizontally and cannot produce vertical force. Red pars are supposed to be 3D printed, the total volume 141 cm3.