Plugging the open end while awaiting new cell

I have the RJ30 system. I am receiving a new cell in about 3 wks. I removed the present cell and need to cap the opening of the housing. None of the hardware places have the correct size or thread. I did purchase at Home Depot a rubber boot with clamp and have that over the open threaded end. I'm not really that comfortable with having rubber for that long. I would like to have a one-piece cap that would screw on but, cannot seem to locate such a one. Any suggestions?