The box building material
I used 3/4″ thick MDF sheet as recommended by @123toid in his build video. Alternatively, you can use baltic birch sheet for almost double the cost comparatively but a lot less messy to work with.
I purchased my MDF sheet from Lowes. They will cut it for free or a very nominal charge. Home Depot also offers this service. You have to be careful though – not everyone in those stores is a trained woodworker and their cuts can be off the mark. So, I had the saw operator cut the MDF sheet in a slightly larger (rough cut size) of 16″ x 21″ size. The actual size of the front and the back panel of the speakers is 14″ x 20″.
The rough cuts made hauling the MDF board a lot easier. Besides, cutting the rough cut boards to the precise size was a lot easier using my consumer grade table saw.