Don't use rubber! Install the tensioner and tighten it down good so it will not move at all, then bend it to align it with chain wherever it falls on your frame. (if possible, sometimes it just won't work.)
I usually find they need a bend towards the wheel, and a twist to line things up. I use BIG crescent wrenches and LOTS of elbow, but heating it cherry red makes it easy to bend.
Eyeball it ON the frame,then remove it and bend it OFF the frame until it lines up
Or, if you have access to a welder, do this:
Cut off the slotted section of the kit tensioner and weld it on the frame.
If more is better, then too much is just enough.
If you can't afford it, build it yourself.