A great gift for sure! I have to agree that there is almost no possibility of finding an original cover for one of those, but it can’t hurt to check with Weber Australia. Since it is a flat top grill, it might not be as hard (expensive) to have a cover made to order.
We would love to see your Flat Top! Can you post a few pictures in the gas grill section? As Stephen said, these were pretty uncommon in the US, and by now few remain.