I found this helps create a flat surface to keep them stacked a little better. (littlehomereloved.com)
A higher coil count can also create a flatter surface and a more even distribution of weight. (howtoadult.com)
Dried currants (raisins can be substituted), slivered almonds, and dried apricots fill the gaps between the apple chunks and additional apples on top create a flat surface for the pastry to sit on, which gives the tart a nicer shape when it is unmolded. (bookpage.com)