How about this approach. Instead of a traditional folder approach which in some Finder views can show document contents — wrap the folder and contents as a disk image. The Finder sees it as a solitary file without a disclosure triangle to reveal internal documents. When you want access to the image contents, you double-click it, and it mounts on your Desktop showing the document contents. You would unmount it from your Desktop with the Finder eject command.
Disk Utility gives you control over whether the created image has read-only, or read-write access, encrpytion, and you can even suppress the .dmg extension to avoid tipping that it is mountable. You can even replace the disk image icon with an image of your choice.
Disk Utility > File > New > New Disk Image from Folder… choose the relevant folder and select the image button. Determine the name and Save as location. For image format, you can ignore the last two options as you are not going to CD/DVD. If you choose encryption, make sure you maintain a backup copy of image names and passwords on removable media. It will run for abit, so don't think it has hung on you.
Once you have the file.dmg, press option + command + I (Show Inspector), and optionally, suppress the .dmg extension. Leave this window open. Open an applicable image in Preview, and press command+A to select it. Then command+C to copy it. In the Get Info window, Click on the small icon adjacent to the filename. It will get a blue border. Press command+V to paste your Preview image into this icon. There will be a small pause before it appears. Close the Get Info window.
Now, double-click on your new disk image. It will mount to your Desktop showing the included Numbers documents. Depending on your choice of read-only, or read/write when you created the image, you can now access these documents as appropriate.
