Mockup stirling engine need help

[Bricolo07]

Hello,
I just walked the forum and the web for explanations on the behavior and operation of a stirling engine (model).
My final goal is to use it in a solar application, either in a parabolic mirror or stuck to my thermal box, but we don't have it there yet, not not.
The desired step is to have a functional model, by following tutorials on the internet I created two small motors, one in metal can / box and another thin flat waterproof box.
https://www.youtube.com/watch?v=RzW0--FMbVg
https://www.youtube.com/watch?v=X2qssmxA82U

My questions because I have some ignorance is doubt:
On this kind of engine (gamma), should the displacer be perfectly sealed or should the air pass easily?
How to make the system waterproof? At the top must necessarily make a hole to let out the rod of the displacer so there is a leak.

In the videos, at startup we see that the user is pushing hard because he encounters a small resistance and op it's gone. On the best there is no friction so when I throw it makes 2-3 easy turns but there is no resistance or momentum.
An idea?
I could make you photos but from the outside it really looks like the ones in the video
Thank you