The only time I've build a heat shield was in Nov year ago when I went kayaking.

I used an old camping table that I fliped over. Now I knew it is going to be below freezing overnight and few of us wanted to hang out in front of the fire without staying too close to the flames. So... My situation was kind of similar to yours as far as the water is concerned. First of all we couldnt form a 90 degree angle on the table top due to a bench being attached to it so our reflective surface was at about 60-70 degrees which we figured out is better because it would circulate the air. So we tried two things:

First is that we put the table on the side of the fire (6 feet away) that was away from the water. Small breeze was going over the fire and hiting the heat shield that reflected the heat back at us and whoever was at that side of the fire was very warm. Whoever was on the water side was still freezing from the water. We fixed that by laying down a kayak accross and that kind of trapped the heat. Hey even few degrees made a difference.

Second was that we positioned the barrier between us and water trying to kill the breeze and cold wind. That worked fine and gave us kind of even heat all around. But again our backs were little cold (exposed people complained less thou) but people with their backs to the shield said that heat is not as great as in the first trail.

If it was me alone and I had to reflect the heat I would have used the first method and use breeze as a free fan.

Now how do I know that our stuff worked? Well water that was left outside the parameter of the fire was frozen in the morning. Water in the cups that was left by the heat shield was in its normal liquid state. Like I said couple degree difference was not much but enough.

Now I wouldn't use my space blanket for the shield but look for other stuff such as wood, branches, scrap, whatever but the question is: if you head one blanket and nothing else would you rather build a heat shield from it or wrap yourself around with it.

Obviously I'm leaving out other ways of keeping warms such as building a bed with hot rocks underneath, dig ins with heat vents, etc etc...