I'm so sorry to be the rain on the parade , but car seat manufacturers will null and void any warranty of safety and function if anything is placed/attached to the car seat that didn't come with the car seat. This includes those newborn head rolls, mobiles for the handle, extra seat covers over the exsisting, etc. I feel obligated to throw this in here because many people do not know this and if anything (God forbid!) were to happen these manufacturers will say "too bad, but you used after market items". If you ever get your car seat and base inspected by a certified car seat safety inspector (through the Police or FD) they should also tell people this.
Although your idea is lovely and looks great!