We've just had to have our Safe and Sound carseat altered to fit our daughter who is now in a spica. We went to the Royal Children's Hospital Equipment Department and got an extension strap to reach around her middle. Then we pad the seat with blankets so she is actually sitting on the seat, instead of inside it. She's still secure, it just looks really awkward, but that's what our surgeon told us to get. The staff at the RCH (Melbourne) came out to the car to see what was needed and then helped my husband install it.

Hope it all gets easier for you,

Corelly