Child seat: how to protect my child's face from rain and wind?
Hamax certainly make a dedicated cover, though when I had a Hamax seat I used a cover from another brand (Raleigh I think). Both keep the child's face uncovered.
My worry with that design is that with plastic sheet in front of the face you really need a sufficiently sturdy support to hold it clear of the mouth and nose, even if the child falls asleep and the wind is at its worst (despite the plastic being fairly stiff). But the support mustn't be a hazard in its own right, for the rider or child, even in a crash. I'm not saying it's impossible, just that it's more challenging than you think to make it safe. You'd probably have to destroy the rucksack to obtain parts, unless you had something very similar to dismantle