Spotteds are in fact excellent climbers in my experiance. I was fooled by their diminuative size, and ended up searching my entire back yard for one that escaped a pen my woods and boxies could not in all the time I'd had it.
A lip around the top is a GOOD idea. As well as predator protection if this is an issue in your area.
Avian predators can be detered by netting or fencing or plenty of lines or cords above the pen. Mammals can be deterred with an electric fence wire or a barrier they cannot cross. Deep water will also let the turtles escape from mammals that will not go into deep water after them.