It's all made of 3/4" MDF, cut with a circular saw and a homemade guide, and the sides on a tablesaw. If you get a plywood blade, this works best for MDF and makes a clean cut. I think you should build it to the width for your specific trunk. I didn't actually measure, just used the actual depth as a guide. What I did was first roughly lay out the bottom piece, then cut it and laid it on the spare tire below with some 1/4" spacers so I would have some leeway from the tire. Then I measured from the bottom piece to where I wanted the top to be, and cut the sides. The top was then made using a bandsaw and cleaned up on a sanding wheel.

Everything is put together with glue and screws, and the top is attached with screw-in inserts (McMaster-Carr part #90192A117) that are epoxied into the sides. This gives me a good way to attach and remove the top repeatedly (for amp level adjustments) without damaging the MDF. Also, when screwing the MDF, it's good practice to drill the hole just slightly smaller than the threads on the screws (and longer than you'll need), use straight shank metal screws, and countersink the both the head and shank locations. This picture illustrates why:



When you tighten the screw (even a bit) it will raise the edge of the MDF enough to split it unless you are careful. This method works well for me, no split edges after using this for years. The glue will seep into the holes and make the joint even stronger, too. Don't use a drill to drive the screws, or you will definately split the MDF if you are not very careful. Split MDF makes a horrible joint.

I reinforced the existing metal piece in my car with some aluminum stock, cut to size. The aluminum can be cut on a tablesaw with a regular blade (buy a cheap one just for aluminum cuts) and drilled on the drill press. The hinge is a piano hinge from a hardware store cut to size and riveted to the aluminum stock. Then I attached the hinge to the rack with screws. Take it step-by-step and go slow, you can get it done. Here are some more pics of the amp rack:









How did your first rack fail?