Once you have completed the six outside pieces, you are ready to add the inner middle shelf. Measure the inside distance at the joint of the widest part of the shelf. The distance for the 28 cm (11 in) sided shelf is 41.5 cm (19.375 in). Make sure your measurements are very accurate.
Using this measurement mark, cut the middle shelf piece the same way you marked and cut each side piece (steps 1–5). The middle shelf piece is a longer version of the side pieces, with 30-degree miters cut at each end.