How about an 8'x8'x2' tall box on the lawn around a 6'-diameter x 2'-tall
EPDM-lined welded-wire fence tank with a 300'x1" pressurized plastic pipe
coil and a 2" polyiso board cover with EPDM over that and a fountain pump
that moves water up through a shallow draindown pond when the tank temp is
less than 130 F and the EPDM temp is greater than 130?
The pond could have a greenhouse polyethylene cover, with a approximately
parabolic reflective north wall made from foil-faced foamboard above that
and an 8'x12' single polycarb south wall.
On an average December day in Atlanta, with 1110 Btu/ft^2 on a south wall
and a 50 F daytime temp, this might collect 8'x12'x0.81x1110 = 86K Btu
and lose 6h(130-50)8'x8'/R1 = 31K, for a net gain of 49K Btu.