I don't know of a particular manufactured plug but you should be able to just cut a length of wooden dow rod to the appropriate size and insert it inside the magazine spring. That's all I ever used for the plug on my old Ithaca 37. The mag still functions fine but the dow keeps the folower from travelling too far and accepting additional rounds.
The only thing I can't remember is if the end of the Defender's mag tube is open or closed and if there's room to drop a rod in. I can't recall if you can just drop the dow in or if you'd need to fully disassemble the mag tube body to insert it.
