I've seen those "Cami Secret" dickies for sale at Walmart, and I was drawn to the idea of having extra coverage without having to wear a full extra layer under my low-neck shirts (because that's the last thing I need when it's over 100 degrees outside). I was confident that I could make one myself, and wanted to improve the design to prevent the dickie from bunching up. I used an old stretched-out cami, and just cut out the front part. I attached snaps for it to attach to my bra straps, and elastic to go around back to keep it flat against my chest. I didn't bother hemming the edges, because I'm lazy and it's just knit anyway. The finished product looks totally weird.
But it functions beautifully.
After finishing this project, I discovered that Downeast Basics sells cropped camis specifically for the purpose of making v-necks more modest. I bought one in black and one in white. I'm not sure which option I like better (mine or theirs), but I think the bottom line is, if I hadn't had the sewing supplies on hand in my stash (an old cami to cut into pieces, sew-on snaps, elastic), I probably would have just went to the mall and bought my solution off the shelf!