I used a baby Bair vest:

You can buy them at bumps to babes (about HK$250 from memory). They aren't recommended for car travel, but I figured they were better than nothing. I live in DB and it was impossible to carry a carseat into town with me.

The vest goes over the baby's head and there is a loop which you thread your seatbelt through. At least it will stop the baby going through the windscreen. I used it until my daughter was about 1 year old. Now she sits by herself in the seat belt.

Hope that helps.