Assembly instructions:
Glue the button to the upper part of the spring and insert it into the body, then close everything with a plug. The spring has pre-modelled supports and it is a little bigger due to material memory so after some squeezing it gets the right length.
(The spring is not my model, the author of the model is HeyVye
Model link:
https://www.thingiverse.com/thing:4591444)
Link to the projekt:
The author marked this model as their own original creation.