I just bought a new 2017 Mirage less than a month ago, and I think the trunk set up is a little different, but I was inspired by this thread to make something similar, and ended up with a nice box for about $20 in materials.
I started with some of those rubber foam interlocking flooring mats, since they're very light and I don't have to worry about them scratching anything. Home Depot was selling them for around $17 for a pack of 4, and you only need 2.
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I used the manufacturer's carpet to trace a basic template that I then trimmed down with a utility knife to get the base size for my box. You can see the tracing a little easier here on the plywood that I used for the lid:
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I used some lightweight 1"x3" boards to frame the box, which has sort of a flattened hexagon shape. My initial measurements were 38" for the back, 18" for the two long sides, 12" for the two angled sides, and 14" for the front, although I think I ended up making some additional trims to get a nice snug fit. Using the 20° setting on my miter saw seemed to work pretty well for all the boards that were placed at non-right angles. I glued everything together and put in a few finish nails, and used long staples to tack the rubber foam mat to the boards.
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For the lid I just used some scrap plywood I had lying around and I covered it with some brown canvas that I bought a bolt of when the Hancock Fabrics here went out of business. If I had gone out to buy materials for it, I would have picked a lighter board or used plexiglass, but the price was right on this.
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I used a couple of hinges to attach the lid to the back, and covered the bottom edge with a line of tape since the rubber foam was still shedding a bit from being cut with a dull knife. I also used spray adhesive on the masonite board that the manufacturer used to cover the spare, so that the board will provide more support to the foam mats and it will be easy to lift up the whole box if I need to access the spare.
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Everything fits nice a snug when the back seats are folded down, and there's just enough gap with the seats up to put the manufacturer's carpet back on and tuck it down over everything.
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And now when the seats are down, it's a flat, continuous surface to the back gate.
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