The description below gives all the necessary measurements
Start with some foam core
4 pieces 1" by 6"
4 pieces 4" on top, 6" on bottom, 2" tall
Make them into a box, sort of shape
Make lid for box, 6" square, and rabbet cut the edges
Make 4" square lid, and rabbet cut it, for slanted box thing
Glue both boxes together and while that's drying make a 4" cube with rabbet cut lid
Glue that together
Add Slanty parts
Center piece is 8" long by 1"
Side pieces are 8" on one side, 5 1/2" on opposite side, 2" along bottom edge, and 1 1/2" wide.
Add Corner Pieces
Corners are 3/4" squares, rabbet cut, and joined