Just added a second shelf to the back of our 2013 Fit.
I searched for 'double trunk' on the forums but doesn't seem to be what I did.
Looking in the trunk, I noticed about 12" up on either side edges that a shelf could rest on.
The area is almost exactly 4' long and 16" wide resting against the back seats.
A stainable pine board 4'x16"x 1/2" was the starting piece from local home stuff store.
Using foam board, (two pieces taped together in the middle), I cut the shelf to size with a notch on the left side.
I put this on top of the pineboard and traced the notch with a marker.
THen a few seconds with a jigsaw carved it up.
Also put a notch on right side to leave room for the metal bracket the magic seats attach to. another photo
I used a flat file to slightly angle all the edges to reduce future splinters.
Then covered with 1 roll contact paper on top, and on edges. If you spray adhesive medium on, just the edges, helps them stick. Didn't need to spray board because contact has own adhesive. If you cut carefully can have it fold down around the notches and cutouts.
Then sprayed top with medium adhesive onto contact paper and put one roll of grip liner from 98cents store over the shelf liner contact paper. I baked it in the sun for 15 minutes at 72o to dry the spray adhesive and voila is finished.
Finishing touches were small self adhesive foam rubber strips for the ends so the board doesn't mark up the plastic side of the fit where it rests.
If you cut this board right, it will be slightly hard to fit into place.
Be sure you can fit it into the desired spot before you start covering it or you might have to cut the board some more.
Angle the middle notch end (left) in and up, set the other end down on the shelf then lower the end notched in the middle.
Now you have second shelf in middle of storage area 12" up from the floor of the trunk, with a good amount of room for stuff and junk below the top cover.
Beach towel went over the grip liner in case we have a remote picnic table that needs covering.
Then significant other found three bins that fit in this space to hold stuff.
the net is a U-shaped cargo net for keeping grocery bags upright.