I've had this file up on Steam Workshop for a while now, but I recently decided I should probably also Nexus and post about it here too, so... here it is!
This Dawnguard mod (yes, it requires Dawnguard), sets all containers in both Fort Dawnguard and Castle Volkihar to never respawn. Without this mod, most of the storage in Fort Dawnguard will reset in 10 days (most of the ones in the Castle were already safe, but not all of them). With this mod, none of it will, but the rest of the zone will still reset.
This means that dead bodies will be removed, plants will reset, items on tables will restock, and so on, but all storage will be safe- making both great player homes.
I may add other small tweaks to this mod to make both better player homes, so if you have any ideas, feel free to suggest them.
Downloads:
It's available for download from the Workshop and from Nexus, see the links below and use whichever one you prefer:
http://steamcommunity.com/sharedfiles/filedetails/?id=88484125
http://skyrim.nexusmods.com/mods/24130/