A monolithic lens-window-prism (LWP) device, made of lithium fluoride (LiF) or magnesium fluoride (MgF₂), was proposed. When either of the devices was fixed onto one end of a gas cell filled with Xe, it becomes a “wedge-crystal”-like device and was used to convert a 1 MHz femtosecond 347 nm laser to its third harmonic radiation at 10.7 eV. This led to an improved beam profile and a more compact and less lossy configuration. A stable output power of ～11 μW was demonstrated for 2 h using LiF-LWP. In addition, MgF₂-LWP was also verified for its practicability at 10.7 eV.