I suspect the two requests will be mutually exclusive....
If you have a proper Triumph rack it has a sticker on that says max load 3kg. (I know it'll take more, but I'm trying to be resposible - and then no one can blame me when they've overloaded it and it's broken off!!) To have a bag that is big enough to be a back rest when not on the rack, that's gonna weigh about 3 kg in it's own rght!
I have a couple of bags, both are from
www.jafrum.com and while I haven't used my enormous one (I bought them while I was on a downer and waiting for the insurance claim to be sorted out so I could buy the 06, and it was at the end of the rally season in the UK - retail therapy...) the little one I got that does fit on the rack I'd recommend to anyone. I don't just use it - I abuse it. I carry my general crap in it to work and then load it up with my work kit and use it to carry that around... it's about 8 kilos in total. and it's still ok after 6 months.