His bookcase of choice was the cheap and very hackable BILLY. However the BILLY is not modular, unlike the PLATSA system which lets you position frames like a step bookcase.
The only solution was to hack.
Admitting he was not skilled at carpentry, Costantino kept it simple.
His plan: cut the two sides into different heights and forget about a sloped top.
Here’s what happened on first attempt. It looked promising.
And the second.
Sloped ceiling bookcase hack
First, measure and measure.
Then, he marked the cut lines on the BILLY side panels. Be careful to correctly mark the left and right panels. Cut the side panels with a jigsaw. For a cleaner cut, he recommends using a blade with finer teeth, set at a slower speed and to exercise more care in removing the masking tape from the cut lines.
And after completing the first bookcase, place the upright of the second BILLY side by side with the first and mark the cutting line. Otherwise you may get two sides with one or two millimetres difference, which would be a pain to fix.
Without the top panel, the BILLY was wobbly. Costantino shortened the top piece and positioned it lower as a top shelf.
The last step was to cut the back panel. Measure the top position at each side, draw a line and cut with a cutter on the white side (not the brown).