To truly make it read-only, the cheapest solution would indeed be to write the data to a DVD-R or CD-R. A folder setup with my instructions is read-only, but an administrator can change it even if they have no permissions to the folder. Although doing this isn't something someone could just accidentally do.
The other option would be to buy and install a hardware write blocker, this can be pretty expensive. There is another way to do it, perhaps when I have some time I will create a tutorial for creating a read-only virtual partition.