This is how I built my 3'x6' can solar heater
  The size was determined by the size of the plexiglass
I could find the day decided to try to build.

Materials
1  4'x8' sheet 3/8" exterior ply wood for the back
1  4'x8' sheet 5/8" exterior ply wood for sides
1  4'x8' sheet  1" foil backed insulation

2  tubes High temp silicone

4  cans of high temp barbeque flat black spray paint
1  3'x6'  sheet 1/8" Plexiglass

AC fan  110 Volt AC  100 to 130 CFM
1 Snap disk thermostat that starts the fan at 110 degres F
and turns the fan off when temp drops below 90 degres F