I've found that both outside and indoors (in the dry air of a heated house in winter), large containers stay moist a lot longer than small ones. Plants in small containers can be nestled down into large containers, effectively underplanting a small tree or other plant for a combination planting that also helps keep the small pots from drying out (they'll wick up water from the larger pot, even if they aren't terra cotta, especially if you shove them a couple inches down into the soil of the big pot).

My favorite "trick," though, is using polymer moisture crystals. They are similar to the crystals inside disposable diapers (which you mentioned), but they will last 2 to 5 years. Diaper crystals are meant to break down in a season, and they do. I get mine from https://watersorb.myshopify.co.... The medium is the all-purpose size. Powder is good for making a "slurry" to dip bare-root plants before shipping or planting. Fine is useful for pots under 4" (think seedlings, african violets, succulents). Large is good to mix into the planting hole for trees and perennials (the larger the crystal, the longer it persists in the soil).