The design and structure of the tub is just as important as the materials used to make it. Many manufacturers use a segmented inflatable design, reinforced with a vertical I-beam structure inside. These internal vinyl panels make for a sturdier tub, one that can support users leaning against it or even sitting directly on the outer wall of the spa. Premium portable hot tubs add another level of stability and protection with interlocking panels that surround the inflatable tub. Made of waterproof composite materials, these panels provide a rigid wall that protects and insulates the hot tub.