Width of GTV and spider both 1780, so would think the boot width will be the same, roof racks usually adjust for width anyway.....It can't fit it much worse than the GTV it is shown on! The boot area is much bigger on a Spider so it should fit better. The only variable might be the boot width but I would be surprised if different to the GTV and it is probably adjustable for width anyway.
I would put in a question to the seller to check width and adjustment. It may even say in the original instructions if he has them if also suitable for the Spider.