Keeping autistic son in his car seat!

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Our ASD five year old has taken to undoing his seatbelt and we're having real problems getting him to understand that this is unacceptable.

We've ordered a "seatbelt guard" but it doesn't get particularly good reviews and after a week it still hasn't arrived.

Anyone know of a better harness type system that would fit on to a Britax car seat, or even a new car seat that has a harness designed for older children, not just infants?

Help appreciated!