This is a general purpose linear servo actuator (pusher style). Two sizes have been designed, for different space constraints and force outputs.

These use continuous rotation servos which helps keep the cost very low. Off the shelf actuators of this type can cost around $70 USD.

The “mini” version will fit in smaller spacers, but has a much lower force output. The “large” version has a higher force output, but is…larger in size than the “mini”.

Here is a video that shows how to assemble and run both sizes:
https://youtu.be/2vAoOYF3m8U

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Some inspiration for this design came from:
https://hackaday.com/2016/03/31/3d-printed-case-turns-servo-into-quality-linear-actuator/

Included in the files are a list of materials for each size, as well as affiliate links for the non 3D printed parts.”

Link