There's an unused bracket with hole hanging off the water pump. No idea what it's there for but it might be a good mounting point for something (although the hole is kinds big), generic bracket or adjustable rod.
Just turn the coupling center one way, both ends go out/tighten belt, turn it the other both retract, loosen belt, I put a jam on it to keep it still. It was $3, give me a break, I hate the box, kick the box! If I can score a cheap alt with the right mounts I'll try it if not I'll get back in the box.The adjuster rod you first referenced had Heim joints at each end... so it can move/pivot without undoing the mount points... the one you suggest using will be rigid mounted at either end, which means you have to loosen both ends BEFORE adjusting - not worth it.
Just turn the coupling center one way, both ends go out/tighten belt, turn it the other both retract,