What is a quick fix for a keyless chuck with a missing retaining clip?

my black and decker cordless drill (CD18SR, Type 2, 18 volts) is missing the retaing clip that holds the bit release ring in place and keeps it from falling off. the drill was my dad's (may he rest in peace) and i need to know how to fix it, being as I'm the one who uses the power tools around the house the most.. if anyone has a quick, cheap, and somewhat simple way of fixing it without costing too much $, that would be much appreciated. thank you all