Is there an actual slot on the bag for the seat belt to slide through? If so, then this is a good method. You ideally also want some kind of tether from the bag to your baby.
It is my understanding that using the crate is not the best method of transportation as the crate needs to be buckled in, but in the event of an accident your baby could still be thrown around inside.
There are several types of doggie car seats available. Sissy uses one and many here use them. I feel for us, this is the safest way to transport as this is the sole use for the item.
__________________  Sissy & Angel |