On one of the blogs I follow, the writer lamented her inexperience with making cushion covers with zippers. For some reason, and I don't know why, I can't post comments to a lot of the blogs I follow- it's very hit and miss (does anyone else have that problem?), so I thought I'd give her advice here, in case it's helpful to anyone else.
I'm a long time cushion maker. I made the cushion for this bench, from tracing out the shape on an opened paper grocery bag (which I still have, case I need to make one again), to cutting out the foam in the shape of the bench deck, to wrapping the foam in poly quilt filler, to sewing the whole thing up.
The advice that I would give Julia, is that when you're making a zippered cushion, sew the zipper in first, just like you would put a zipper in a garment piece. Then pin the sides together with the zipper closed and stitch those seams. Then unzip the zipper and finish the rest of your seams. Once all the seams are sewn together, pull the cover right side out through the open zipper, and voila!
For this cushion, I used two invisible zippers, but I needed them to run along the entire back of the cushion for ease of stuffing. They didn't quite make it, so I left the gap, which I tacked closed once I got the cover on the cushion. You only see it if you're looking really closely at it; otherwise it disappears.
I hope this information helps anyone out there thinking about making their own cushions - it's really not that hard.
I mean, I figured it out.