Agreed with everything Kobain's Mommy said. The whole premise of the side curtains on the Advocate are for the people around the seat, but with them it is very hard for people to fit around the seat, it's kind of a catch 22. Personally, If I like the cover on the Boulevard better I would just buy a Boulevard. Chances are if you ever need to get it in a tight back seat situation it will fit better anyways. (vs. the added bonus of the side curtains).
ETA: The main concern with a custom cover isn't actually the warranty voiding, but the fact that it has not been tested with the seat and you have no idea how it will act in a crash. Maybe it will work and your child will stay safe, maybe it won't... and it's just too much of a gamble to take.