Hi, I just had a thought and figured I'd run it by you experienced seamstresses/upholsterers. I am making cushions for my porch furniture, don't have very much fabric, and can't find a coordinating fabric I like. So could I "cheat" welting on a box cushion by making the seam allowance on the top (and maybe bottom) seat piece(s) a little larger, "pinching" some clothesline along the edge (leaving enough on the outside to sew the seam), sewing it all around so it kind of stands up, *then* attaching the boxing/gusset on the outside of the clothesline? Kind of like this, skipping the separate steps of making the bias strips, covering the cord with the fabric, and then sewing it on. Starting with the "attaching piping", see how seat piece is right-side up, I'd put clothesline under following the edge, kind of fold it in towards the middle of the cushion, use the zipper foot on the outside and leave plenty for SA on the outside. Then continue as in the tutorial to attach the gusset, using the sewn-in piping as a guide.
Here is a link that might be useful: Cushions with welting