Initial test shows that the outlet temperature (where the tin metal is) could reach 21 degrees celsius around 11 am even though I have not insulated the long outlet pipe. The ambient temperature was 2 degrees above zero.
I used Polar Bear Eavesdropper Eavestrough Foam as the heat absorbing material bought from HomeDepot (Note added on Nov 22: DO NOT USE THIS STUFF! When heated, this gives out some smell and one plant which got the blow directly withered because of this – use metal or fiber glass materials instead). Between the box frame and 1/4″ plywood, I used the 1-1/2″ Durofoam board which has an R value of 7.5 and for the sides I used two layers of the 1″ Durofoam board (R=5 for each layer).