I've made this with those tools, but it's slow and unprecise.

I should emphases that geometry is complex. I am practicing making a car from blueprint and there's a lot of triangles.
Easiest way to do this would be to select edges and then chamfer. This is not the only place where I need to do this .

As you said, edges must be coplanar. One more thing, angles between faces aren't exactly 90°.

TIG, can we expect some plugin maybe?

Native Offset requires selected face. I have something like in attach, but much more complex.

Ok, new challenge .

Is there any plugin which works like ChamferAlongPath, but to preserve original edges (I don't need bevel). I just want to add some geometry which will be parallel with original edges from both sides.

Heh, but you did it in 1 min. .
Thanks again.

Excellent tool for objects like this. You'll see some geometry distortion because I rotated few cross-sections .
Sketchup is became very powerfull software .

Hi Gulcan,

That looks quite easy, I'll try it, thanks .

Here's results. Texturing is done with projected material. Not bad I think? Base for sampling is rounded box (pic #4).
How to map shapes like this?

Bedspread problem solved, thanks .

@thomthom: I think that is nice to have some thickness on the edges, but it's not must have .

One solved...one to go.