Every few years I move all my files into a disk image and start from scratch. I need the image to be (mounted) read-only for safety and searchable.
So far I used sparseimage, made Spotlight index them and then locked the file.
This time I tried sparsebundle as it was "recommended for 10.6" and can put images on file systems that don't support tens of gigabytes in a single file. However I can't find a way to make them read-only, no matter what permissions I assign to the bundle or the bands folder inside.