The tensioner you linked to is essentially exactly what you have now. It wouldn't be any improvement.
Move yours down to the chain stay (the lower frame rail) and secure it. There are many options for doing that or you might want to explore what others have done in the way of a tensioner bracket. Bridging the chain and seat stay is the best way.
Your bike does not have horizontal drop outs so some way to tension the chain will need to be provided. A tensioner is one way or some will suggest shimming the engine. I'm not a fan of that method because the engine needs to be securely mounted to the frame and shims, if not correcly installed will allow the engine to shift in the frame. That's not good.
Take some time and read the discussions on tensioners and how best to deal with them. There are many pages on the subject.