Just finished a new, improved sandbox. The prior one was irregularly shaped, hard to cover, and lined with a tarp so when it got wet it would stay wet. The new one:

• Added a couple boards to make it strictly rectangular
• Treated plywood lid in two pieces with handles, to make it easy to cover in a mostly waterproof way
• Lined with landscape cloth, so if it gets wet it can drain, without letting in worms, ants and such from below

Motivation source: Starting a new (temporary) day job soon, so I could afford the materials, and would have little time to work on it.