This is something that will be frequently laundered so I'd stay away from the pellets. Good idea but not real practical, in other words. I have a pattern that I got when RSVP was active several years ago. It is pretty much like Rob's but the neckline comes around to be fastened with velcro. The nursing home where I inquired about their need for these mentioned not to have the neck too snug. You might ask a local nursing home for a sample of what they are using and get more ideas for a pattern. Personally, I'd stay away from the pellets because of frequent laundering.
By the way, I've heard it is more acceptable to call these clothing covers, not bibs, because of the infant connotation.