Help with linear actuators

What you are seeing here is the interior of a robot my students and I are conceiving. The robot's name is Wall-E . My question to you is: I want to use 2 different kinds of linear rails.... The horizontal ones will be 20x20 and the vertical ones will be 20x40.... Why don't I see Linear actuator bundles for 20x20 rails and only see them for the 20x40 rails with either a nema 17 or 21 motor?

 

Thank you