I just finished a fire pit. I built it up with brick, and filled the inside with pea gravel. There's a drain at the bottom. The ring sits on the pea gravel and is covered with lava rock. They sell pans, but they are very expensive. I don't see a need for a pan. Why can't the ring just sit on the pea gravel? Rain will just quickly filter through to the drain at the bottom.
The link below is a good resource.
DIY Fire Pits & Parts | WoodlandDirect.com: Outdoor Fire Pits, Outdoor Fireplace Units, Fire Pits, Outdoor Fire Pit Accessories-Woodlanddirect.com