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Should the connection between the ground and my hotbox be sealed? Ok not sealed but should I fill the gaps with dirt?
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Sorry you never got a response. Yes the idea of a hot box is to hold in the heat that is generated by the greenhouse effect, solar energy getting trapped in there. So you would want to pile dirt around the bottom to keep cold air from coming in.

Thanks rainbow, this is exactly what I did. But since then it hasn't been below freezing to have to close it up. Not that I'm complaining though!

I guess this means also the top should be sealed to the base.. It still ices at cold nights

I guess you have a coldframe from what it looks like. Its very nice! Using a double plastic or double windows on the top will work better to hold the heat. A little tip that I am doing this year is putting several gallon or quart jugs of water in the corners which will help hold heat from the day and help cool it when its too hot. One of the biggest probems is if it gets too hot especially when the outside temp is above 50 with the sun out. .
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Another thing is paint the back wall white to reflect the light back onto the plants. I have about 8 or more cold frames of all sizes. I use one for a compost that I will cover with a foot of dirt more near the spring and plant my first lettuce crop in! All my cold frames are basically raised beds that I use for both cold frames in early spring and raised beds in the later spring!