When I originally bought the Vulcan, I chose to get the passenger backrest to give it some style and to make it more comfortable for Chanya. I really had no idea she wanted to get the CB650F at the time of purchase and she only rode on the back once. Although a top box for the Vulcan isn’t a factory standard option, there was a number of styles I saw around the web. As for availability in Thailand and something I could check out before buying, the options were somewhat limited but the 320SP bike shop had a good range of options.
320SP
320SP Website
https://www.320sp.com/