We usually take those AM tensioners and drill a hole through the nylon pieces and just bolt one half (as in one nylon piece) to the frame somewhere along the length of the chain. I wasn't able to find a better picture of it but you can see the top of the piece in between the 2 middle wheels.
About the zipties, we have actually used them as tensioners before in very desperate situations (like at a demo when we forgot everything else) and they work for a very very short time before the chain cuts through them. We have however had chain run against metal (1/4" bolts) before and it usually is fine.