My 2014 was difficult. Didn't seem like enough room.
Put the bottom in first. You have to slip it in between the housing and that big glob of crap behind the housing whose only purpose is to make your life miserable.
I used my cell phone as a camera see what I was doing.
Once the bottom was in I could push the top in until it locked.
It helps a bit if you swing the handlebars to move the tire out of the way.
This will allow you to wrap your hands around the frunk and touch the top with one hand, and the bottom with another.
Either that, or you have to remove the frunk.
It took me about 20 minutes of slow, careful tedious trial by error, until I got the cap back on.
I found that it helps to lubricate the work with verbal references to BRP, all the BRP engineers, the unmarried parents of the BRP Engineers and frantic calls for help to various deities both real and imagined.
It can be done, but what a PITA.