Re: Is there a Canadian version of this site?
(Assuming the OP comes back...)
The tensioner design you're using could probably user a shorter, stronger spring. The should reduce the problems with the arm making contact with the chainstay. If that's not an option right now, moving the anchor point for the spring you are using forward might do it, to get more tension on the spring. Looks like you have a little room there to work with. Instead of using a zip-tie, a hose clamp would be a more solid connection point for the spring to the frame, and it won't travel or stretch out like a zip-tie.
To keep the arm from swinging all the way down and contacting the chainstay maybe you could drill a small hole in the upright bracket, and put a small bolt through it to create a stop for the arm to make contact with first. Put it through from the inside (so the head of the bolt is on the wheel side), run a nut or two up to the bracket from the outside nice and snug to hold it. The remaining length of the bolt ought to stop the arm's downward swing.
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