Photodiode sensors should be covered for optimum tracking performance. The recommendations for sensor covering can be found in the following documents that should be part of your Steam account:
C:\Program Files (x86)\Steam\steamapps\common\SteamVR Tracking HDK\training\presentations\090_sensor_covering.pdf
The recommended materials are on slide 16.
Additionally, the material covering the sensor should have a diffuse finish. Fully Transparent (non-diffuse or specular) will degrade sensor performance due to internal refraction. The recommended surface finish to make the above materials Diffuse is MT-11020. They may find this specification in a reference drawing located at:
C:\Program Files (x86)\Steam\steamapps\common\SteamVR Tracking HDK\design\mechanical\outer_front_housing\300-003474-rev-a.pdf Sheet 5