Ok, a thin strip cut out of the plastic (don’t let the color confuse you) bottle works fine for keeping the mudguard erect. Probably a part of aluminum can would have been even better for this purpose.
But while this significantly reduces the amount of grime on the backpack, it does not eliminate it completely.
Maybe because the mudguard is not aligned with the wheel (as described by Biggsy50 in the first post in this thread) - is it common problem or is it designed that way?
I’ll experiment further and update…