Thank you! I will update on the progress.
The box I got here: Slät låda 144x82x30 (L x B x H) ABS [ce660] - 35.00:- : Swech Trading. I had to cut out the display myself and if you look closely you can see it is not perfect edges. But it's homemade so it doesn't have to be perfect :).
I really like the idea of morse code. I will defenetly look into that. The furnace already have lambda sensor controlled air intakes. Otherwise I would probably have done some tinkering there as well:). What I have been thinking about is to add a function to lower indoor temperature at night. The control system I have for that does not have that function.
I will look into putting the code online somewhere.
Thank you for the feedback.