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I have read that some say the shock cord cuts in if you only wear a T shirt. I usually have a collar.
New Zealand is cold isn't it?
I thought about making one, but with the clips etc I considered...

If you want as something simple look at Rick Young Outdoors /Bino Harness.

I can't find an Australian distributor but they freight anyway.

He also shows several methods of slinging his harness...