I have a 400s, and two 650 Exec's (11, & 12 model years). All are fitted with a Bestem 2012 T-box.
Both the 650's were an easy fit, but both had the backrest.
If you go the Bestem route, they are good value for money boxes, but do not expect the quality of a Givi unit. The boxes themselves are fine, and I've had no leaks in several years of use, but on 2 out of three of mine (and the third is only a week old!) I have had issues with the clearcoat, and had to respray.
On one, the clear yellowed, and flaked off in areas, and while the other had no flaking, the entire area yellowed within 5 months, making it seem like my silver bike had a gold top-box!
Both came back up well after being rubbed back and sprayed (changed color slightly on one before clearing), for less than $20.
Also be wary of the Bestem mounting hardware. On my 2011 (box purchased around a year ago) it came with spacers, which leveled the support plate, but without hardware, so I had to source my own bolts and washers. I also had drill the third hole in the support plate.
On the 2012, (box purchased last week, fitted Saturday) the "Burgman" box was no longer available, but a Burgman mounting plate was. So I purchased this as well. The mounting plate came with spacers, and hardware, just unfortunately not the right size. The spacers did not level the plate, or box, and would have interfered with the tail unit. So I had to space up the rearward items using washers to level. But after this, the supplied bolts were too short to mount the support plate.
So, I just fitted the mounting plate, then drilled another couple of holes in the support plate to align with the mounting plate holes, and used the supplied hardware.
On my 400s, (box purchased 2010) I had to drill the tailpiece, but the hole positions were clearly marked from underneath. So, not a difficult task. With this one, I didn't get mounting hardware (other than the brackets to mount to a rack, which all come with) so I used bolts coming up from underneath, with washers and nuts above to hold in place. I then used nuts and lock washers above and below the support plate, to allow me to adjust for level. Once done I ground off any excess bolt.
None of this is difficult, just don't expect a straightforward bolt-on with parts supplied.
Most difficult part of fitting any of them was wiring the light into the brake-light circuit.
I hope this helps.