After printing Lobocnc's awesome Desktop Stomp Rocket I found that the plunger likes to slide down on its own inside the launcher (damn you, gravity!) so I conjured up a quick spring.

I only made one full circle which should be printed on the print bed but will be the top side of the spring in the model. This I did because I wanted the spring to take up as little space as possible to have as much air pressure for the rocket to launch.

The first spring I designed was thinner, but the plunger was too heavy so it compressed the spring by its own weight.

Printed with supports everywhere in the “snug” style of PrusaSlicer (you can find this in print settings)
The supports where okay to remove for me, but be careful not to break the spring.

Edit:
User AtticStatic wrote in the comments that they printed it successfully without supports