I made one of these... Cost me 86 cents for a box of velcro from Wal-Mart cause I already had the fabric and coordinating thread:
Shouldn't void the warranty, the fabric doesn't interfere with the handles, and I won't have to worry about it falling on the floor, but it can easily be flipped up in the car or at restaurant to see baby. I made it out of fleece fabric so it was a little heavier for winter. I'll probably make a lighter one for spring/summer. I used leftover fabric to make some little car-seat-sized fleece blankets to tuck over the baby too once he/she is strapped in.