S.B.
We did the blanket/jacket prop thing, too. Sometimes I would even stick my diaper bag in there to sort of wedge her in a sitting position. Those cart/highchair covers are nice, we just never invested in one. This phase is pretty short-lived. He will be sitting on his own before you know it! Also, some places have better highchairs than others, with higher backs and better buckle systems...you might look for those when you go out. We ended up buying a portable seat to take with us, it had more stability than the highchairs and it folded up so we could take it with us wherever we needed it, even after my daughter cpuld sit by herself! Good luck!
S.