A #3 zipper should be good for you. Thru-hiker also sells them by the continuous foot for pretty cheap. I've played just a little bit with both kinds of zippers...it was tougher to get the slider started on the coil zipper, but not really a problem. But if they'll install it for free, might as well let 'em do it.
Why not just use one zipper in an "L" shape, with two sliders?
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