Parametric windowsill planter with reservoir

A rectangular planter box with two purposes: one, it's resizable so you can exactly fit a windowsill or other narrow space; two, the reservoir is designed for watering by wicking, so the soil can fill the upper box and little water is lost to evaporation. I wanted to maximize the soil for plant roots on a narrow windowsill.
As in the pictures, feed something that wicks (cotton string) through the drain holes of the upper box; fill the upper box with plant and soil. Water the plant by fillin