It's easy to make for about $25.00, you need 2"x3/8" of pure copper, 2"X3/8" of Pure Zinc, These two cost about $20.00, and also 4"X 3/8" of medical Latex. You may have to purchase 1 foot of copper and a 1 foot length of Zinc. This will cost you about $2.5 for the copper and $18.00 for the Zinc. The latex tubing is about $1.5 a foot. Plus shipping costs for the Zinc.
Cut the Copper and Zinc with an hacksaw about 2.5" lengths, polish them both with wire wool, cut the Latex into 4" lengths. Push the copper in one end and the zinc into the other end of the latex tubing. join the copper and the zinc with another length of latex. And bingo there is your ROP.