Windows in a log cabin

[d000hg]

Most people don't know that log cabins 'breath' and can vary in height a non-trivial amount over the year. This makes fitting windows difficult - you cannot affix the side of the window into the ends of logs, and you have to leave an expansion gap at the top.

Our manufacturer recommends this 'fascia sandwich; approach: https://www.tuin.co.uk/blog/adding-a-window/#Final_Fitting_of_the_Generic_Window

We are using UPVC windows and doors so are building a wooden frame first around each window, then have the same issue to solve.
The fascia method definitely works but it is not very secure. If you remove the outer fascias, you can just push the window into the room!

I was in two minds if this is a windows or doors issue but wondered if some fitters or smart carpenter has done this before and has any thoughts on it?

There's a couple of other things it raises, let me summarise the issues:

1. Windows/doors are not secured except by the fascias, which can just be unscrewed from the outside
2. We worry about water getting in behind the top fascia board in driving rain and then running in/around the window frame, somewhere we cannot access.
3. For very tall windows, or french doors, you would (I'm told) really want to attach it at the sides to keep it rigid... but we can't.

For reference, we are warned to allow for up to 40mm total change in height over a year. Not a trivial amount.

 

[d000hg]

I'll say my ideas for people to critique (or demolish!)

• You can of course screw the units in at the bottom providing at least a little stability.
• Let's picture the basic set up, and how to affix at the top to stop someone pushing the window out:

The window (black and grey) is mounted into a wooden frame (red) using screws and then sits in the whole in the log wall, with an expansion gap at the top.
We cannot constrain the movement.
So what if we have a kind of dowel type affair:

We drill holes up into the log(s) above the hole say 100mm. Then we put some sort of rod through the wooden window frame fixed in place, sticking up. Like say an M12 bolt but the hole in the log is wider than the bolt so it can't engage the thread.
We make the bolts long enough they cannot fall out of the holes, and the holes long enough the bolt will never hit the top.

• What about at the side? We can't do the same idea with the bolts but we could rout a channel down the middle of the window, and put bolts through that are held in the channel, a bit like the way your wheel holds the tyre in place:
  

Are these sensible or stupid ideas? If not totally idiotic, how would you improve them and in particular, what fixings might I use? Are there things out there more designed for such things than "a bolt that's not as big as the hole"?!