Stinkopants....if I understand your original post, what you're really trying to do is give yourself a decent trunk with a backrest, mounted in place of the passenger seat? And you're hoping to do this with stock mounts, reasonably cheaply and soon?
If so, Bill is correct - you really don't have any options that don't require some fabrication...
However, "fabrication" isn't necessarily a bad thing here. It can range from building your own trunk with upholstery and mounting it (search for some of Playingotter's past postings there), to figuring out a way to use the existing pillion mounts to bolt the new trunk to (my backrest how to's might help there)....or, actually doing some metal work to create a rack on which you can mount the trunk. The level of complexity is probably more in your control than you think.
I will make a suggestion based on my work on creating a front seat back rest for the GV250 - if you take an existing trunk with a backrest as you have mentioned (eg., the one at Amazon which you posted earlier), then you can probably make it work with the existing seat mounts on the GV250 by using some longer bolts, some "L" brackets, rubber spacers, etc....but also expect to do some drilling of new holes in the trunk and adding some other brackets.
Will it look like a custom job for an extra $30? Uhhhh, no....
Will it be functional for that?....Probably, if done correctly.
(My "trunk", for both the GV250 when I had it and my Suzi 800 now, has been a "Rubbermaid Cooler/Carrier", sold at Walmart and "Sam's Club" for about $25 - and I've had to modify it to put on hinges, mount it, etc...look for any of my posts on "trunks" to see what I've done. And I had to spend about $40 and a Saturday to make a rear carrier on the Suzi to mount that $25 trunk...but my other options would have been about $150 for a "Cobra" rack to fit my rear back rest, and half a Saturday. Would have looked better, but accomplished the same thing...)
Are there better ways to handle it? Yep, if by better we mean cleaner - I'm not a great "fabricator" (as many of our posters here will be happy to point out...
), but I am pretty much a "form follows function" type of guy. Most of my "mods" over the years have worked, but aren't gonna win any fabrication awards - and I could throw lots of money at all of 'em and make them far better looking.
Point of all of this? You can make it do what you need and how you need it...quickly, cheaply, beautifully....but not necessarily all of 'em at the same time.
You'll have to decide which of these are your priorities as you look further into it.
One last piece of advice, though....the trunk that you posted about is not gonna provide you a whole lot of storage. (A half or 3/4 helmet is the max - mine holds a full-face and some beers!) You might want to go with a larger one if you're also removing the back seat (which is where I've been carrying my "all important 12/18 packs" prior to putting the trunk back into play - now I can carry a 30 pack or two 18 packs again in the trunk, plus one still on the rear seat!
Lots of luck, and let us know how it goes....
You pretty much summed it up, yes.
OK then, so I guess there's no "safe" trunk to buy for my purpose; I'll just have to grab one I like and hope it can be modified to fit.