I am a car seat nut and what you posted, I would use personally, it's thin and isn't going to interfear with the harness any more than the thin fleece coats and layers techs recommend over coats.
I would have loved that for DS because he was forever kicking off blankets walking into stores and whatnot.
This baby will be older by winter so hmm? Maybe just use the carseat poncho we already have.
Proud SAHM to Connor (8-6-10)
and our little