A cheap zip-up hoody from Primark, removed zip and stitched the sides together from the inside leaving a deep V-neckline.
The eyelets I salvaged from an old corset top (the plastic boning had warped hideously!) which I pinned and sewed on, though equally you could punch holes and fit eyelets straight into the top.
I made a bit of a sloppy job on this one, at some point I'll get round to neatening it up and improving the fit. It is very cosy afterall.