Check out this method of securing the chain tensioner with 3/4"
hdw store aluminum angle and zipties.
Also, most bike frames the rear frame tubes are not parallel with the chain.
So the idler won't be parallel with the chain without some tweaking.
I tighten my tensioner bracket good. Then use a large cresent wrench just below the slot, to tweak the bracket parallel with the chain, then install idler and check alignment by eyeball.
Hope this helps someone