I can't recommend idler sprockets enough, they're easy to work with, reduce your efficiency losses, and aren't too heavy.
Here is a picture of our implementation of them on a u-bolt, this design can easily be mounted on some aluminum angle and can be used sideways (although the chain might be a bit more likely to fall off, but some sort of spacer on the u-bolt could solve that problem, if it occurs) a simple nut can adjust tension, or they can even be spring loaded, as I suggested
here .