baking soda and vinegar should work fine I had been thinking about that for a while.
My thinking was to put some vinegar in a 2 litre bottle with a few holes in the cap or around the shoulder of the bottle.Then add some baking soda.The holes should limit amout of CO2 coming into atmosphere so the plant doesnt get too much at once.Using this method you vary supply by either covering holes or making them larger depending on what your plant needs.
Please excuse me if that got a bit confusing.....these ideas always seem simplier in my head