I used to use 2 3L soda bottles for my DIY setup and plumbed it into the intake of my Rena XP1. It worked great until I tired of the inconsistency and messy refills of the bottles.
I remember when I first started keeping aquatic plants I bought this setup that had a rectangular plastic container that went into the tank and you attached it tongue wall of the aquarium with suction cups. Then there was another smaller container that stayed outside the tank that had a tube running to the container inside the tank. You would put the co2 tabs (pretty much alka seltzer w/o acetomenaphen) in the outside one and then close it and it would fill the inside container with co2. You would then wait for the co2 to dissolve in the water, it was pretty much like a bell jar. Looking back at it, it was quite a joke. My my the rungs I have learned through out trial and error in this hobby!