I built one out of some scraps that actually turned out pretty nice. I just found a 4" wide strip of 3/4" birch laminated plywood. Then just miter cut the edges at 45 degree angles and use wood glue and brads to assemble it. After we put the brads in, we drilled holes right next to the brad and put in some screws. The screws were countersunk into the wood to allow for plugs and it also hides the brad indentation.
To make the plugs you take a scrap of hardwood, I chose ebony, and then use a special bit and drill out dowel like plugs that then fit into the countersunk holes. Glue the plug in, then chisel the excess away and sand.
There are probably many methods out there, I just liked the look of birch and ebony plugs for my amp box. I didn't make a bottom half because it didn't need one and I drilled a 1/4" hole for the wire to come out. Click on the first thumbnail