Slats are a great option. Slats should be the same width, flat and spaced at an equal distance from each other, no more than 7.5 cm apart.
Keep in mind that the Leesa mattress should be set up on a flat, rigid surface. Some slats are bowed or flexible which can change the way a foam mattress is intended to feel and aren't recommended. Using bowed or flexible slats with your Leesa mattress can cause damage to your Leesa mattress.