That looks awesome! I've been thinking about doing this for a while now, I would love a write up about this!
It's really pretty simple. The way I did it is not the best, but it was pretty cheap. I started with a 6x6 piece of sanded plywood, traced the existing trunk insert. If you have a band saw, it makes cutting the plywood extremely easy, however, I just used a skill saw for the straights, and a hand saw for the corners/turns. For the actual design, that is completely up to you. I just went to Home Depot and picked out a cheap box of laminated hardwood, some wood glue, and a small role of felt insulation (just a little something so the wood would damage anything). I found it easier to piece the laminated wood together, and then glue it to the plywood, rather than gluing piece by piece. (Note: make sure you have enough weight on top of your wood when glueing so that the glue doesn't dry in thick streaks and cause gaps.) Make sure the glue is completely dry. Afterwards, cut off the overhanging laminated wood and sand the edges, to your liking. Attach the felt, plastic or insulation, if you so choose, and insert in your trunk!