I have used the timber battens as support for the planks that will support the slide board.
First I did measure how many I needed to a get a good feeling. Not to far apart so it became a "bumpy" slide because that the planks was to far away.
I decided on my tries to have a space of 140 millimetres between, its a bit bumpy for 79kg. Even if that would be to far away apart for a child I will just add one board on top of this when its worn out so that will be a fix by it self I hope.