This is how my tray was cut

 

Okay that makes sense that way. But...

Is it hard to fit the tail back on with the 1/8" (or whatever it is, just a guess) of black plastic that is the stock undertail (the part that fits against the subframe) PLUS the thickness of the new undertail?

The stock undertail (again, whatever you want to call it) curves up into the tail while my new ACTUAL undertail is more level/ actually slightly dips down towards the tire. So If I were to cut the stock black plastic out (all except battery box) there would be a massive space under my passenger seat. But there would be no holders and everything would slide to the bottom. I don't really like the idea of having the old black plastic in there (stock) if I don't need it. Is there anything wrong with cutting it out all the way to the battery box? (given that I leave enough material to overlap slightly so there are no gaps)

I know there were more questions but I forget