Make a sensor mount by downloading the attached Corel file and laser cutting your 1/8 piece of acrylic using your awesome Epilog laser cutter.
Did I mention how awesome Epilog laser cutters are?
Anyhow... If, unlike us at Instructables, you do not have a laser cutter, you can download the file and print it to paper. Cut out the circular outline from the sheet of paper and tape this to your acrylic as a guide. Where necessary, drill holes through the acrylic using a power drill and cut around the outline with a jigsaw (or similar).