What you're asking for is whole-disk encryption. And it must be compatible with NTFS. So google NTFS whole-disk encryption.
Then it also has to be compatible with Mac OS X (or with Tuxera), so add that to your search criteria.
If you can't find anything, then I'd say a lock-box with a key is your only remaining alternative. Or go to your local bank and ask to rent a safe deposit box. Take measurements of your disk drive, because smaller boxes cost less than larger ones, and you'll want to rent the smallest one that fits the drive.
Also, if you do find some encryption software, I recommend making a full backup of your disk to another unencrypted disk first, and only then testing the full-disk encryption with tuxera. Because it would be really bad if the software turned out not to work, and you lost all your data in the process of trying to secure it. That's one advantage of a lock-box: zero software compatibility problems.