Carve the cover, using a 6mm bit and a 40mm thick board.
I used hard maple, but pretty much any hard wood can be used. Possibly soft wood as well.
If you don’t have access to 40mm wood, glue together some thinner pieces, like two 20mm boards.
If you use the ½ inch board from the Inventables store, glue together 3 layers for a 1½ inch thick result and reduce the material thickness in the project to 38mm.
Don’t go below 34 mm thickness, since that will leave only a 2mm thick bottom for the cover.
Make sure the glue is well and properly dried, and the glued surfaces are completely smooth before gluing.
If you’re not using an external speaker, I recommend adding some rubber feet under the cover, to allow the audio from the bottom speaker to get out easier.