I know there are wind guards for motorcycles, specifically to protect the hands of riders. There is also a product called "hippo hands", which is a sort of insulated bag that goes over motorcycle handles and encloses the rider's hand completely, that really helps in cold weather. Unfortunately I was unable to find anything along these lines for bicycles. So I thought I'd make something myself: it might look goofy, but it does the job.
My office mates (some of whom also ride bikes to work, and suffer similarly from the chill) have been admiring my handiwork, so I thought I'd share the design. The tools and materials for this are basic, and you may already have them laying around the house (I did), but if not, they are cheap and easy to find.
Step 1: Tools and Materials
Two large plastic jugs. These can be from juice, iced tea (which is what I had -- my roommate likes the stuff, so I always have some sitting in the recycling bin), possibly even milk (though I thought clear ones would offer less distraction for my peripheral vision)
Two long zip ties (they have to go around your handle bars, all the cables, and have some slack for holding the jugs)
Two small screws for mounting
Duct tape, to cover the cut edges of the jug so they don't cut your hands
Exacto-knife or box cutter (optional, but very helpful for starting cuts in the plastic)
Hand-held hole punch
Step 2: Cut the Jug
When that's done, look at the top side of the jug, where the mouth was. You need to mentally divide it into four or five roughly equal sections. Use the hole punch to punch holes through the plastic between these sections, as deeply as you can reach. Then use the permanent marker to draw Vs connecting these holes to the top. Basically, what you are trying to do here is make a sort petals arrangement, which will help the top of the jug be more deformable, to conform to your handlebars and any cables and make as tight and secure a fit as possible. The reason you are punching holes first is so that the cuts end in a blunt shape and don't keep "running", potentially ruining the structural integrity of the whole thing.
When you are done cutting out petals, use the hole punch again to punch out holes in their tips, leaving a safe margin of plastic around them so that you don't think it will be easy to tear.
Step 3: Add Mounting Accessories
In order to make the wind guards stable, you will need to secure them to your handle bars with more than just a zip tie around the top. Hold them up where they will be mounted, so that they enclose the handle bar tips, brake handles, and whatever you have on your handle bars, while leaving a space for your hands to enter. There's usually a hole in the grips at the end of your handlebars. Use the permanent marker to mark where that hole lines up with the bottom of your jug. This is where the screw will go. Use a sharp implement to start the hole (the tip of your exacto-knife, or an awl will do, or you can use a drill). Then take the screw and screw it in a little bit.
Thread your zip tie through the holes in the "petals" around the mouth of the jug. You may need to partially pull it out and thread around brake and/or shifter cables before tightening it.
Step 4: Mount
Line up the screw in the bottom of your jar with the hole in your handlebar grips, and screw it all the way in. Then adjust (and possibly re-thread) the zip tie around the handlebar and cables so that your hands and wrists fit inside without pressing against the sides of the cutout. Tighten the zip tie, and trim the end. If you haven't covered the cut with tape (or glue, or Sugru), adjust the cut for maximum hand comfort, and then make the edges safe. Repeat this on the other side of the handle bar. Take your bike for a small test ride to make sure everything is comfortable, and make adjustments as needed.
Now enjoy still having feeling in your hands when you get to your destination!