http://cccpastefzuz6unowzgjtdwmxwjfgf5za7owwci3b2zys4jmc3xjszad.onion/cccea7c18ebdb7dd
You may not need to make a custom image: try the OEM image first (OpenWRT GL.inet for a GL.inet mango).
The extroot can be anything that block can mount. Currently block creates some restrictions on what extroot can be. It must a filesystem of type: ext2/3/4, f2fs, btrfs, ntfs, or ubifs (note that it can not be a FAT16/32 filesystem).