I used 1/8 in acrylic, but the precise thickness varies from sheet to sheet, so you'll want to make some adjustments.
There are 6 layers that make up the case around the sensor. They are numbered front to back. Once you have the 6 layers cut, measure the thickness of one piece and the thickness of all 6 layers with some calipers. Use those measurements to make adjustments to the thickness of the "mortises" on the TOP and BOTTOM pieces and the overall distance between their outside edges.