This is a case for the optical heart pulse sensor from pulsersensor.com. See http://www.pulsesensor.com for details.

The case should build easily, without extra support. Before assembly, clean up the wire channel, for instance with a suitably sized torx driver. Insert the two outer wires one at a time through the channel before inserting the centre wire. The sensor itself should snap in place. You may add a dab of glue in the wire channel to provide some strain relief.