Hi telnoi, I think you'll want to create a kind of rounded surface shape for each side, probably sweep is easiest to use for this.

If you take 2 curves that cross each other like so:



You can then select one of them, run Construct/Sweep, then pick the other curve as the rail path, and that will create this kind of surface:



I think that one is a little too rounded, you want something a little flatter than that. But that should give you the idea hopefully - you can control how round it is by the shape of the original curves.

If you want curves on all sides, you could take that sheet and then copy it around to all 5 sides and then use Construct/Boolean/Merge to fuse it all together and extract out the solid in the middle of them all.

So that's one method, I think another way would be to use the Rail revolve command, I'll try that alternate way in a minute here. (EDIT: looks like Pilou has already shown the rail revolve method, thanks Pilou!)

Also I guess you could just use Revolve instead of sweep to build a rounded cap surface similar to the above.

Once you have a rounded cap surface like that, you can use Boolean difference to slice a solid with it to leave the rounded imprint in the solid.

- Michael