Hi: 3 0z is pretty thin and any wear do to corrosion and your dead. It would have to be an open loop drain back so the only pressure it sees is due to head resistance and just the weight of the fluid. There have been many attempts over the years to get away from the expense of Cu. Most of the ones that work half decently are in the hot air category rather than a liquid based unit. If money is a real problem then a black fiberglass matt style air collector might be the way to go and use a liquid to air exchanger to get the heat into your DHW. You could keep the blower small to raise the output temp a bit and use twin wall poly carb for the glazing, AL sheet metal sides and back, etc.. That's probably the cheapest solution you can get that will work half at all worth talking about. If it is for a typical res DHW sit, I would go about 100 SQFT of collector and 80 gallons on the preheat tank.