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:

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

STL quantities to print:

Mini version-
Motor Bracket, Mini Version {QTY 1x}
Pinion Gear, Mini Version {QTY 1x}

Large version-
Motor Bracket, Large Version {QTY 1x}
Pinion Gear, Large Version {QTY 1x}

Pusher- (works for both types, choose only the desired length and print QTY 1x)
75mm Long [gives stroke of ~50mm]
100mm Long [gives stroke of ~75mm]
125mm Long [gives stroke of ~100mm]
150mm Long [gives stroke of ~125mm]