Why not just tie the male belt in a knot or two over itself to reduce the length? Snug the knot as far toward the connection with the seat frame as possible so you don't have a big knot bulge near the car seat.

It sounds like you are using a forward-facing seat if the belt is going across the kid's back? Ideally, according to the "powers that be", with a 9 month old you'd use the infant "bucket" seat anyway, where the belt would go over the top of the bucket (the connection will be suspended in air then, at least that's how it landed with ours), or rear-facing seat where the belt is going across the bottom of the seat (instead of the back), but I assume this has been considered and isn't possible.