Neither do I, but it’s pretty easy.

You have already inserted USB stick or SD in EXT4 format, (I format it using DiskGenius in windows 10)

  1. Go to Luci>System>Mount Points. Firmware 3.201 or 3.203, before it does not appear in Luci.
  2. Click Add and in General Settings, add the SD card or USB stick, I use the first option, UUID, you can see which it is by the size of your device if you have more than one in your router. I have the 512Mb stick and 2Gb SD NTFS format card (to share files with my whole family). As you can see in previous picture.
  3. In MountPoint below select Use as external overlay (/overlay). Also check Enabled on top but I haven’t checked now and it works fine after reboot.
  4. Save and Save and Apply.
  5. Reboot
    Check in Mount Points that your device is now sdba1 /overlay if SD card or sdb1 /overlay if USB stick inserted.

My 512Mb old USB stick has more than enough space for plugins I need.