Well after messing around with the design one thing that might do what you want is whenever you have a "\\" in your code you could put a "_" there and that should make it look like the line is going through your snake. For the bottom you probably would have to do something similar and put it between two "\\" 's in your code so it shows up between them. I think this would help but I do not know for sure saying I myself am just starting to learn C++.
