Yes. It helped to have a hacksaw, with the metal rack I had to trim one of the mounting bars down to get it to fit neatly. Not sure if the resin rack on the L2 has the same problem because the bar was already trimmed down by then.
If you have problems - as it "This looks impossible" , unbolt the rack , fit the top box plate then bolt the rack back down. Probably easier as tightening the nuts underneath is a bear anyway.
I got my 46L Bestem top case yesterday. Thanks for the advice on the hacksaw, I definitely needed to trim some pieces of the mounting bars to get it to look neat. I also had to hack off a bit of the ends off the provided bolts to get it to fit on the 2012 DL650's rack, at least the way I installed it. And then it was a bear, as you put it, to get the nuts tightened underneath (forget about those lock washers). Fortunately I had the right tools to jury rig a way to do it.
Like you, it only occured to me after doing all this that it probably would have been easier to drill holes in the "universal" adapter plate provided to line up with where the holes were on the bike's rack, and then just to bolt the plate onto the rack without the brackets, than to saw off pieces of the bracket pieces. And to visit a hardware store to get M8 metric bolts in a shorter length if that's what I needed... But I wasn't sure my drill bits were strong enough to drill through the metal plate, and I had a hacksaw handy (as well as a few spare M8 sized bolts with nuts that were shorter than the ones the case came with).
That said, I really like the box. I haven't wired the brake lights yet but the capacity is perfect - I can fit my entire set of riding gear (pants, jacket, boots) and still get the lid shut. Which means in the summer I can get off the bike, get comfortable (put on sneakers or sandals) and not carry anything with me when I get to my destination! And in colder weather where I would be wearing my jacket and boots off the bike, I can easily fit pants and helmet in there no problem. (Gloves go inside the helmet)