I managed to find the time to solder the board. Ive run into little problems with voltage setting of the LCD display but everything is fixed now on the schematics. For some reason i thought i can apply 0 V on VO pin of the LCD to get the perfect text brightness- I was wrong. The voltage has to be slightly higher. So either i would have to use DAC (that atmega8 devices dont have) or solder a pot somewhere (what I did, its under the LCD).

As for the DS18B20, i have soldered it on a cable and put behind the window:

Its covered by thermo-shrinking thingy so its quite waterproof. 

Whats to be done now is to add some additional software features like turning the backlit of the LCD in the night etc. to save-up some energy. Also the PC application has to be extended to add more logging functionalities and plot drawings.

Below I add a small demo video shopwing the soldered board.