I think the shad top box rails bolt straight on without modding the panels and they're about £66 on evilbay. Some bugger butchered my rear panel to fit a givi rack and made a right arse of it!
I looked for a decent alternative all over but they're mega bucks, but being a tightarse I ended up making my own.
I rarely use it, and just have the small seat bag attached for the usual crap that won't fit under the seat.
As for bolting the box to your pilllion seat, well each to their own, but I'm not a fan of it, I think it looks awful.
Stop polishing it and ride the bloody thing!