Based on a bit of googling, the balance of opinion seems to be that whether you choose to widen the platform on the carrier or not is a matter of choice, but that some form of ratchet strap is the usual solution.
Would it need to be something like 25mm ratchet strap rated for 250kg loads, or would something like a luggage ratchet from go outdoors be usable? Or in a pinch, a ratchet style jeans belt or two?