I'm not sure how many people this would apply to, but I figure a lot of people like me are relatively new to the hobby and on a budget, so don't see buying much equipment anytime soon.
Anyway, I recently picked up a 6 gallon carboy at a resale shop (definitely my best find ever at Goodwill!) as a companion to the 6.5 gallon one I had already, and I wanted to try a secondary fermentation step - mostly for kicks. Since my current batch is 5.5 gallons, there's quite a bit of headspace, which I didn't really think about ahead of time. It's already in there, so oxygen is unfortunately probably already doing a bit of work on it, but oh well.
I've been thinking about next time around perhaps trying to "fabricate" a CO2 blanket for the secondary, and different ways of doing that. My options seem to be: a) buy a smaller carboy specifically for secondary, or smaller primary batches; b) buy a compressed CO2 tank, which would work nicely with a keg system down the road - not really there financially, though; or c) make my own CO2 and get it into the carboy somehow so that it can settle under the oxygen layer.
I'm liking c) until my discretionary spending can widen up a bit. There's a few household-ingredient reactions I'm looking at to get the CO2: so far I can think of Alka-Seltzer tablets in water or vinegar and baking soda (very similar reactions, I know). Question: are there any more efficient CO2-producing reactions I can get with cheap, commonly-available ingredients? For getting the gas into the carboy, I can see using a dish in the large funnel/strainer set I already have, with a cover on top and positioned over the carboy opening. Also, if I can get a pretty good reaction going in a small-mouth container (think small Erlenmeyer flask), I could fit a balloon over it to trap the gas, a la middle school science project; it would be pretty simple thereafter to direct the gas into the carboy.
Anyway, even if all this turns out not to be strictly necessary or efficient, it still sounds like fun and I think I'm going to mess around with it. Any suggestions/admonishments are welcome!