Hi Slobodan - here is a way to avoid the flat bottom, attached as sink_6.zip.

What I did here is I took the model from sink_5.3dm above, and deleted the flat bottom.

I drew a couple of curves off to the side there, to make a sort of shallow curved surface to use for the bottom instead of the flat bottom. In this case I used a sweep with one rail to make a slightly curved piece, then centered it on the side piece and used Construct / Boolean / Merge to cut them with each other (Edit/Trim would also work), and then deleted the extra pieces. Then I used Edit/Join to join the remaining pieces together, and finally a fillet with a blend cross-section shape puts in the rounded portion along the bottom.

I think this is likely your best bet for making a clean construction here - don't try to model too much all in one shot, do the side parts and then the bottom part separately and let fillet do the work of making a smooth blend in between them.

Let me know if you want more details on any of these steps.

- Michael