Rear Lap Belt

I know it goes against the grain for a lot of you, but has anyone fitted a rear lap belt to a 330ci convertible if so is it difficult or is there anyone who offers it as a afterfit. - I've 3 young kids ( and a wife) who would occasionally like to go out as a family in it. We have a Shogun as a family car so its not a major problem but it would ease my conscience.

Chris

What's wrong with the seatbelts that are already there?