So I need at least somewhere to store my helmet while I park the bike, my helmet doesn't fit in the boot and only just fits under the seat. Also, I like to be able to take off my motorcycle trousers and jacket and leave those in the bike too, so a big 45L box should be just the ticket. It probably won't be full much while actually riding, just a backpack or something (famous last words! :-)
-simon
I must say the question of what helmet fitted into an X8/Xevo used to be a regular post, I had an HJC full face and had no issue putting it in the "boot". Do you have a second helmet or if not when you next buy a replacement it'll be worth checking as it sure makes it easier if it does fit in. I take it you've tried inserting from every conceivable angle (oh err) as you do need to slide it in at an angle!!
I had the larger Piaggio screen, the one that looks like a sail, it was effective but I (like many) did have to trim it down about 3-4 inches to be safe. Sure there's some Piaggio disclaimer about using it above 50mph or something daft. But it is the best option as adding bits to the standard screen won't make up for it being too short in the first place IMHO.
Or as Loony has posted elsewhere get the Givi option.