I made a simple C# console app which launches KeePass on startup
and automatically opens specified database.
Location of DB file and password must be specified in config file.
The config file is encrypted on first launch.
Tested with: KeePass 2.34
Feel free to report bugs and you suggestions in the comments.
Some alternatives:
forum.keepassx.org -> keepassx-autoopen
also ddaydj.blogspot.cz