Hi @KevinHlava ,
In order to find the right rain cover for the pack you would want to take a few measurements with it fully loaded (height, width, & depth). On the website each rain cover lists their dimensions under the technical specs found within the nitty gritty.
For example: The REI Co-op Duck’s Back Rain Cover – 40 Liters has dimensions of 21.5 x 13 x 11.75 inches. Based on the photos alone I think this size would be the best place to start.
Hope that helps! Nobody likes a wet backpack!