I called up DNA in Greece today and after a while they connected to the right person :

Very friendly ! that's for sure!

DNA was indeed planning to produce a cover part for the stage 3 side airbox filter , but this parts is up till today not yet produced .

The reason is, they are not sure it will be ordered enough ,
The only reason for that is the question if there is an actual gain in using this cover or not ...

The cover completely blocks the air intake to the side so this would leave you with a standard side box ,
This option would be helpfull in case of big water crossing , or heavy mud offroad use , you would be able to temporary place the cover , and remove it and afterwards with the side filter
again free .

So in case things will go very bad as the filter side doesn't always get splashes in normal offroad use , DNA prefers me just to place the original cover when things would go rough
and leave it open as long as you do daily commuting and day to day offroad use .

He also explained that the open filter can without any worry be used for general offroad use and can easly handle dust and water splashes as long is the filter is properly oiled
and MOST Important ! that the filter box is completely sealed with grease !!

Even with the original side filter cases and the stage 2 top filter DNA asked me to make sure I put on plenty of grease on the build in gasket of the cases as the airflow
produced by the dna filter easily could suck in dust or water through the original filter cases ...

The side filter is a big improvement as long as you keep it in good health and maintenance there is no risk at all for the bike ..