If you are doing some sort of sandbox environment going beyond just boxes is not trivial. One possible approach is to do like Blockscape, where in addition to the classic box you now have a large repertoire of prefixed angular shapes. I do not like this approach as it makes the interface very complex and frustrating. It makes you forehead veins pop.
So I chose a different approach, where you still lay boxes the same as in Minecraft, but then you can go back and alter them. I saw that a single operation was enough to produce both curved and straight angled surfaces. If you applied it gently you would get curves. If you applied it more, it would straighten out.
Here you can see a round hole and a needle, both initially created as boxes. It only takes a few clicks to shift their shapes:
I think this is the right direction. Still it is not simple to implement. While I got this thing working, there are many issues to fix.
I leave you with a video, let me know what you think: