You can take out a measuring tape and make a whole lot of measurements and use math to find angles, etc and build it out of MDF (found at home depot)
You can also just make a fiberglass box that fits the contour exactly, and make the front face of the box out of MDF. There are plenty of tutorials on how to do this. Just and you will find.
It all depends on what your comfort level is with wanting to learn new things and trying new techniques (and how much you want to spend; a fiberglass box will cost more in material than a box made of MDF)