I recently saw the aluminum can thermal heaters. There was another reference I found to heaters using a platform of river rocks under acrylic/glass. Rocks can hold/store heat for a long time.
I would propose utilizing cans (thin metal heats up fast), and rocks for long lasting heat together in some device. Rocks are heavy, maybe pumice (which can be easily cut to shape/form), or possibly cat litter, or even small rocks. What about used seashells?
Also to cut-down on the "home grown"/redneck syle issues, put a nice sheet of metal over the cans, perhaps with a painted design. I know the cans thermal heater are better when painted black, there`s no reason not to cover it. You could even try to match homeowner`s house paint color to camoflage the heater unit.
You are on the right track. If you succeed you will move from the home-grown units to more industrial units, capitalizing on the materials and components experience you`ve gained. In my area there is a home owner who built a home made carport cover. I need to refer hime here, and watch him get rich!