This 15mm extender part is integrated with 'th_top_clip_x4.stl' and can be securely fastened with screws using 'th_side_clip_x4.stl' onto 'K Extrusion,' allowing for a stable assembly without any wobbling. and for me, another important thing is to ensure it doesn't affect the appearance.

The additional screws you need are only four M3x10mm BHCS; there is nothing else required.

A supports is required as shown in the image below.