Nice fits.
That corner assembly looks very strong with all the different corners.  Shouldn't weaver at all.
Not that you had any problem putting in the corner blocks,  
they looked a little tight when you pounded them in,  (solid) Just wondering if you had loosened up the clamp on the other side if that would have let them in easier, then tighten up the clamp afterwards.  
Neither here or there it's done,  just wondering. Or was it better to have the clamp on so that the pieces are in the position they should be so nothing moves differently?

I've been a SU user for over 10 years,  Always said why change now,  but always willing to entertain learning new if it's better.  Do you like the OnShape better?  How is the learning curve?

I'm not having any trouble with SketchUp Make 2017 other than my warehouse. Most everything else I use seems ok.  I used to have the bezier curve tool,  don't know where it went,  maybe lost it when I changed computers a while back.

Figuring out how to do something you have never done is what makes a good challenge.