You will now need to mount two PMTs: one aligned along the long axis of the crystal and one perpendicular to the long axis. Use the height and alignment fixture that you used when mounting the cobalt to determine the position and direction of the cobalt crystal. (Having a fixed laser pointer makes this rather trivial.) Create some type of marking on the side of the can to specify the center of the PMT.
Attach the L shaped brackets to the OVC can using the blue strap, and align them before tightening down the strap. You do not need to tighten the strap too tight.
Set the PMTs on the L brackets and secure them. One method is to secure them to the L-bracket by wrapping the PMT in a paper towel and then strapping the whole thing down using duct tape.
Attach the power and signal BNC cables. Take note of which PMT is aligned to the long axis (0 ) and perpendicular (90 ) to it.