This 2x Barlow lens "does" more than you might think, as it has three uses: not only can it be used on its own for focus stretching, but the lens mount can be unscrewed and attached to the filter threads of the eyepieces, giving you about 1.5x focus stretching. The part without the Barlow lens acts as a 40 mm extension piece. And the T2 thread, with a suitable adapter ring, allows you to mount cameras.
The image quality of the focus extender lens far exceeds the quality you would expect in this price range. Thanks to multi-layer anti-reflection coatings, it provides a sharp, contrasty image without noticeable degradation in image quality.
Lens diameter 22 mm, fits into a 1.25" focuser.