How to protect my DIY bench power supply from reverse polarity?
You will need two sets of protection and separate the charge input from the discharge output.  If you just put a diode in series with the pack, it will be reverse biased when you try to charge the pack.

Depending on the level of performance needed, you can use a simple PFET circuit (http://www.ti.com/lit/an/slva139/slva139.pdf) or an ideal diode controller with an external FET.  Some of the ideal diode controllers use NFETs which (apples for apples) have lower Rds-on resulting in lower losses (and less heat).