Then a good way, if you didn't like the 3rd party apps, is to use Disk Utility/Terminal like I lined out above:
1. Open Disk Utility and make an encrypted sparseimage from File -> New > Blank Disk Image
- Name it (Save As

to something like MyHiddenFolder (best place is in your Home folder ~/)
- Set Size to whatever you need
- Set Encryprion to AES-128
- Set Format to sparse disk image
- Press Create
2. Use Terminal.app to rename it:
- make sure you are in the correct directory (Terminal should open in your Home folder ~/)
- use the command
mv MyHiddenFolder.sparseimage .MyHiddenFolder.sparseimage to rename it.
- use the command
chmod 600 .MyHiddenFolder.sparseimage to add security
3. Use Terminal.app to access it:
- Use the command
open .MyHiddenFolder.sparseimage to mount the disk image on your desktop
- When you're done and want to hide it again just eject the disk image (drag the disk icon to Trash (which should change into an Eject icon) or mark it and press cmd-E)
Tip: You can auto-complete commands in Terminal, so if you type
open .My and press tab it should auto-complete the command and let you press enter...
