As you can see, I don't post often because I'm new to Vespas and don't yet have a lot to contribute. In this case, however, I believe either I've discovered something useful, or I haven't figured out how to search for old threads that provided this solution...
In any event, I've never wanted a top box for my GTS 300S. Don't like the look. Don't like what I've read about stability. Don't like the cost. On the other hand, I'd love to store my Shoei Neotec on the scoot. When it's hot, I'd also like to store my bulky Aerostich Roadcrafter. Neither item fits in the pet carrier, let alone both. Not to mention I use the pet carrier for other important items.
After thinking about it for the last nine months, I decided to buy a $13 Knog flexible orange key lock, covered in non-abrasive soft plastic, to loop through the face of my modular helmet and the chrome bar surrounding my GTS seat. The helmet sits squarely on my seat when locked, but can't be nicked without cutting the lock, which is not easy to cut. (Check Knog.com) Or perhaps the chrome rail.
When I want to leave the jacket with the scoot, I loop a six-foot Kryptonite cable through one sleeve of the jacket, then both loops through the Knog. Both the jacket and the helmet are then secure (or at least secure without cutting the Knog and/or the Kryptonite).
Even if you like the top box, why not employ this solution, saving your top box for other solutions. Thank you for letting me try to add value to a site I value very much.
