Unfortunately, there is no other way to go around this. Roblox called it ‘smooth terrain’ for a reason, no blocky anything, just rounded and, well, smooth.
If you’re desperate, just make those walls thicker until you cannot see the terrain and cannot see any open space between the parts and terrain. Like this:
Thanks! I just thought about it and it doesn’t look that bad. When a player places a basement, does the holes and the walls leak through so the player can see it on the exterior of the first story. If this is unclear I will send a picture.