The OEM cargo bay floor in the Honda Fit is delicate, unlike this bed liner.
The leaves that cover the rear seats fold back to allow those seats to be deployed. Also, the rear part of the bed liner folds forwards to allow access to the spare tire compartment.
Designed with ribs under the rear part of the bed liner to withstand the entire gross weight capacity of the vehicle, the total weight is still a reasonable 35 lbs for the whole thing.
Please note that I'm only sure that dimensions given are correct for my particular vehicle. I believe they should be correct for Gen. 2 (2007-2013) Honda Fits (Honda Jazz), but I would certainly confirm that this is the case before cutting your stock to size.
In any case, the design should be easily adopted to your situation. This Instructable discusses how to figure out the dimensions for your own project. It is my hope that this makes your project easier and smoother; you may find a blank copy of the drawing, for you to note your own dimensions on, on the last page of this Instructable.
Step 1: Rear Cover
To start, you will need to remove the OEM cargo bay floor. This should be pretty easy: I found mine retained by a couple springy pins; the floor pulled right up.
With the OEM cargo bay floor removed and the seats upright, you will be able to measure the cavity that the rear part of the bedliner must fit in. Measure from the interior trim strip in the back of the bay up to the back of the rear seats. Also measure the width of this space.
These measurements, plus a reasonable allowance, give the dimensions for the size of the rear piece of the bedliner. The total width of the seat-cover leaves is given by the slightly wider space between the wheel wells, while the length is set so that they can be folded back over the rear.
The rear of the bedliner will need to be thicker than the front. To determine how much thicker, I folded the seats down, lay a board across their backs, and measured from the bottom of the board to surface that the bedliner would rest on. In my case, this measurement was 7/8". I then added 1/16", because it was hard to measure and it was best to err too thick.
The size and spacing of the reinforcing ribs on the underside of the bed liner is a basic strength of materials built-up-sections homework problem. If that sentence makes sense to you, note that it may be best to assume that the load is supported by only some of the ribs... this gives a more conservative answer. If the sentence does not make sense to you, feel free to get yourself a textbook on statics and one on strength of materials.
Of course, doing the calculation, I found that I wanted the ribs to be 1 3/8" thick by 1.5" wide - a size selected to be easy to make and to meet my strength requirements - but this was too thick (1.375" > 15/16"). I could've relaxed my strength requirements, decreased the spacing, or made the ribs wider... but the ribs only need to be strong in the center, so I simply made them thinner at the edges, where they rest on the car. (Die grinders: smoke 'em if you got em)
With these dimensions, you are ready to cut the wood. To get the curve at the rear, I traced the edge of the OEM cargo bay floor and cut it with a jigsaw. Try-fit these pieces in the car with the seats up AND down, then glue up the rear part of the bedliner.
Step 2: Hinges
To layout for the hinges, I placed the parts for the bed liner in the car, and marked where I wanted the hinges. In principle, I could've done this outside the car, but doing in inside made it easier to think about the problem.
Installation is pretty typical of doors; a mortise is cut, and the hinges attached so that the panel hinges square to the base.
As far as dimensions go, since the hinges measured .400" thick when closed, they had to be mounted .200" deep - or less, per side - or the plywood would bind and develop high forces when closed; I aimed for 3/16" (.1875"), and it worked out pretty well.
Step 3: Finish
A 1 1/2" hole through the folding leaves allows access to yet another place to put a hook, so it seemed like a good idea, though I have yet to use that hole or tiedown point. The layout technique is as in the pictures: with the bedliner folded, the distance is measured from the hinges, and then the bedliner is unfolded and the measurement transferred to the work.
Finally, to protect the project, two coats of polyurethane wood finish were applied. This makes the surface more durable at the expense of making it more slippery - I feel it was a good trade-off.