When building my Garden Screen in my Outdoor Area, we used the "Whites - Screen Up" system which was purchased from Bunnings. The options for the timber spacers is 20mm and 10mm, but we wanted our Screen to be more private from our Neighbours. I created some thin spacers which worked a treat. Just keep in mind you don't want the timber too close as water will get between and cause the timber to rott. I ended up using the 3mm spacers to create an almost 98% screen out.

We used the 20mm spacers down low, then the 10mm in the middle and the top has the custom 3mm

https://whitesgroup.com.au/resources/brands/screen-up.html