You can create a full clone of your database using RMAN backup/restore, data pump, or any of the other methods typically used to create a full clone of a database.

You should place the clone in a regular (non-sparse) Oracle ASM disk group.

After creating the full clone, make the data files read-only to help prevent accidental overwrite.

For example:

SQL> ALTER DISKGROUP DATA SET PERMISSION OWNER=READ ONLY, GROUP=READ ONLY, OTHER=NONE FOR FILE 'FILENAME';