Ahh, yes! I was going to try and make this with unicode box-drawing symbols but I hit a lack of diagonals of a certain style. Scale it up and use ascii characters! A simple solution.
I was considering using the mathematical falling diagonal character, (‘⟋’, U+27CB) instead of the box drawings light diagonal, (‘╱’, U+2571), but it didn’t seem to make enough of a difference, to skip the extra line needed, so I decided to stick with the box drawing set.