Your question is answered in the thread already, you should go back and read everything, but here are a couple of pull quotes that are relevant:
And as for path manipulation and process:
BUT! I have since changed my method here. Note, this is all in Inkscape.
I don’t use the subtraction method anymore, because of deficiencies in Inkscape. It’s not terrible but I don’t always like how Inkscape manages nodes when you do booleans. Sometimes it’s so bad that it ruins the path and offsetting gets weird, so now my new technique is to do this:
1,2. Make my desired curve exactly as I want it. I started with a rectangle and manipulated the side with the node tool.
Make a copy of the shape #2, and break the new copy #3 where appropriate (in this case, the corners) to extract the actual curve (in red) in question as a separate path object.
4,5: Join the now isolated curve to its mated shape using direct node manipulation.
Flip one piece horizontally so that you can flip the two resulting cut pieces (not always necessary, depend on the symmetry of your curve. It never hurts to do it even if you have the correct symmetry axis, so I just do it.)
Kerf adjust both pieces, and you’re good to go.
For a ton more of info on these techniques and some practical examples, look at my series of posts about trays. In no particular order:
Some other relevant posts that rely on the techniques here in some way:
