I’m not here to offer my services necessarily, but there are already some chest protection plugins that work in some situations. There’s Keys by viveleroi, and Latch by Nighteyes604, but that one needs some updating. PermissionsEX(PEX) already exists as well by zml, you can find it here.
For your Factions like plugin, I am currently in the process of making a plugin very similar called Guilds. I almost finished it, but I had to restart due to some issues. I’m making a lot of progress in the refactor branch, found here. It may not be exactly what you’re looking for, but I welcome suggestions of any kind which could be filed as an issue on my Github.
I know all of these are open source, but the locking plugins and PEX are worth a try at least to see if they suit your needs.