Hmm perhaps Heating CaCO3 to get CaO + CO2, then doing a double replacement reaction with Na2CO3?
Or better yet Ca(NO3)2 to decomposition, because NO2 is useful in certain experiments.
Electrolysis is honestly probably your best method, I've read about cell dividers that were pretty interesting...You would use very, very fine concrete, like a mortar or grout even, and mix it with large amounts of salt.
Then you could bubble the Cl2 into a neutralizing solution (very easy to setup yourself)
You mix up the concrete as normal and pour it into a mold. A cardboard mold shaped to fit your container would be great. For example you can glue or caulk plastic strips on the inside of a bucket to make a channel, then the concrete plate slides in like a window slides down in a frame. The concrete plate should be thin, maybe 1/4 inch to 1/8 inch thick.
Allow the concrete to dry fully, then submerge the plate in water for... let's say a week or two. The immersion leeches out the salt trapped in the plate giving a very porous structure. This porous plate is an excellent barrier for electrolysis purposes.
I am not too sure on the granularity of the salt. It may need to be either very finely powdered, like dust, or perhaps ground a little finer than table salt. The resulting voids need to be big enough to make connecting channels. If the salt is too fine it will be trapped and isolated in the concrete, and if it is very course the voids will allow ions to just pass right through.
I would use that method, with a 5 gallon bucket. A quarter inch thick would be your best bet. Too thin would not stop the ions from passing through, too thick and you waste too much energy pushing the ions through.
If you put the lid on, put the concrete "filter" inside, then added one of the one-way air exclusion valves brewers use it would be absolutely perfect..I think they may even sell the 5 gallon buckets with the valve already built into the lid.
An idea similar to the reaction above is from the "Golden Book of Chemistry Experiments"
Bases from a Salt
In a custard cup, dissolve 1 teaspoon sal soda (washing soda, sodium carbonate) in 50 mL water. Heat slightly. Add slaked lime mixed with water. Stir. Chemical reaction produces sodium hydroxide and calcium carbonate. Filter. Clear liquid contents contains the sodium hydroxide (lye). The calcium carbonate is held back by the filter.
Then it would be smooth sailing from there...afterward I would perform a titration to find the exact pH of the solution however.
Hope I helped.