Quantification of bandwidth for surface waves in a periodic Fabry-Perot cavity-like structure with one grounded plane and one loaded terminal plane is presented. The loads are twin shapes printed on the two surfaces of a dielectric layer. In particular, dog-bone and hourglass-like shapes are considered and compared. Numerical investigations demonstrate the dependency of surface-wave stop-band limits and their central frequency on a parameter that defines the exact shape of metallisation. The considered twin-hourglass structures present a larger band gap with respect to the dog-bone shape, a feature that can be exploited in wideband applications.