I marked seams and unwrapped but I get strange results with long quads making up the bevel - scale is off - (left side of the picture below), while for the beam bevel the scale seems correct (right side of the picture below).
Before you do any unwrapping ensure you have a clean mesh
Remove any internal faces
Remove any double vertices (W / remove doubles)
Recalculate normals (Ctrl+N)
Apply any object cale/rotation (Ctrl+A)
Ensure your seam locations are appropriate
For hard surface models try changing the UV unwrap method from Angle Based to Conformal
Since you didn’t supply a blend file, only incomplete screenshots, no specific advice can be given
Thank you, my post indeed needed editing. I had done all you advised before asking for help, except ensuring my seams are at the right spot (it’s the reason I am posting primarily, I have no idea if this how it’s supposed to be done).