I worked on the Prius mounting setup yesterday.
I put the box in and marked where I needed to put the weld nuts in. Fortunately, on the driver's side that spot fell directly between the frame rails.
On the passenger's side I was also very close to the frame rail, within 1".
So, I drilled the holes for the weld nuts.
And then welded them up bit by bit with time to cool between so as not to burn too much paint and not melt the sealant that was fairly close to it.
A little grinding and a few coats of primer finished things off.
I am interested in suggestions on how to reinforce it a little. MrBigh is correct that the sheet metal is thin back there. I do think that it will be okay the way it is unless the trunk area gets smashed
and there is a roll over in an accident. I was going to weld in some extra metal, but there is a lot of that sealant around the top and bottom (sorry no pics) that I'd like to avoid. There is also the issue of rust-proofing things after the fact (yay Wisconsin salty winters) that is probably going to be a problem. I was able to prime and paint the passenger side weld nuts, but since the driver's side are between the frame rail sides I can't actually get to them to do anything.