This paper presents a proof-of-concept for a local-ization and mapping system for magnetic crawlers performing inspection tasks on structures made of large metal plates. By relying on ultrasonic guided waves reflected from the plate edges, we demonstrate that it is possible to recover the plate geometry and robot trajectory to a precision comparable to the signal wavelength. The approach is tested using real acoustic signals acquired on test metal plates using lawnmower paths and random-walks. To the contrary of related works, this paper focuses on the practical details of the localization and mapping algorithm.