But sometimes things just work differently! I found Gary's blog and is working on the same as I do! So, we could exchange experiences and that really helped.
The first try was with cutting 5mm veneer into slats. But the wood split badly and of course at all the crucial places ;-). I then got 5mm wood strips I only had to cut to the length.
Gary's idea with keeping the slats in place by drilling holes was excellent! I had to off-center them at the top piece because I had to consider depth of the window frame as well as being able to attach a cord to it for lifting and lowering the blinds (not working though like Gary's...).
But instead of making the ladders myself I found some curtain material with 5x5mm holes, hung them with a weight attached at the end. That made it all somewhat easier.
All looked great! But when I wanted to fold them the result was GRRR!!
My curtain material was to thick to fold nicely.
I decided to only add as many slats as I want the blind to be high - this way you cannot really take it up and down ... but it looks great as well. And threading them at the side keeps them in place.