The 4 or 5" of concrete in one end won't be enough weight to keep them in place, and if you tie the coke bottle or milk jug to them they'll probably float.
You need a base or "foot" for your structure to keep it in place and from tipping over. Go to the Dollar Store and buy a couple plastic dish pans to use as forms for your concrete. Mix up and pour about 3" of concrete in the dish pan and stick 2-4 pieces of pipe in it. Let the concrete and pipe set undisturbed for 2 or 3 days, then you can pop the concrete out of the dish pan and reuse it. Or if you wanted to do them one at a time you could cut the top out of a gallon milk jug and use it for a form for a single pipe... but I've never seen a gallon milk jug in Canada.
Adding buoyant material to structure is counter productive because it cancels out quite a bit of your ballast material used to make it sink and keep it in place.
Hope this helps!