I use 750ml wine bottles with smaller amounts decanted to 12oz amber beer bottles. For stoppers, I use plastic reusable wine bottle stoppers and these seem to fit both wine and beer bottles. I also wrap saran wrap around the stoppers to help seal. The plastic stoppers were only a few dollars for a pack of 12 or so at a local wine and beer making supply shop, a 45 minute drive from Lawrence.
I have a beer bottle capper that could be used, but I'm thinking that if the bottle is filled to the top with no air space, any temperature change (increase) might cause the top to pop or leak and not reseal. You can generally tell if a cap doesn't seal on a bottle of beer because it will lose its fizz. Would it be good to add carbonation to developer so you could tell if your bottle didn't seal well?
Dave