Females of the leaf-curling sac spider Clubiona riparia build three-sided capsules, in which they enclose both themselves and their eggs. A capsule is usually constructed by bending a single blade of grass or other leaf twice, each time causing a fold that is perpendicular to the long axis of the blade, and joining the edges with silk. When constructed with monocot leaf blades, the resulting capsule is roughly triangular in cross section and 2–4 times as long as it is wide. We sampled occupied capsules from a 0.16–hectare marsh in central Ontario, Canada. Although we found capsules built with the leaves of cattails (Typha latifolia), iris (Iris versicolor), a grass (Calamagrostis sp.), and an unidentified willow shrub (Salix sp.), for the current analysis we concentrated on the monocots because of their structural similarity. Capsules built on cattails (2.13 ± 0.14 ml) were more voluminous than those on iris (1.63 ± 0.14 ml), and capsules made of grass blades (0.67 ± 0.08 ml) were the smallest. Nearly 70% of the total variation in capsule volume was associated with differences between the plant species. Only among capsules built on cattails was there a significant positive relationship between pre-oviposition spider mass and capsule volume; it accounted for about 37% of the variability in capsule volume. On willow leaves, spiders always constructed capsules with the lower surface of the leaf to the inside of the capsule; and on cattail blades, spiders always made their bends in a clockwise direction. We discuss the implications of our findings for an understanding of the choices these spiders make just prior to oviposition.