I'm working on an image and right now I have it as follows:

So a bunch of lines with square cap ends. The bottom left corner I want it to be curved/rounded. From searching I got the impression the best approach is to join those lines (Path -> Union is what I've done) but then with 'round join' set in stroke style I end up with rounded caps on the ends of lines (corner looks good though) like below.

How should I go about achieving the effect? I can think of really time consuming ways (basically manually drawing curves) but I'm sure there has to be something easier. Thanks!