If you cannot find a suitable model (or geometric shape that matches) online, you could:
- Open a new sh3d file.
- Draw (thin) walls with arc extents set correctly to outline your countertop.
- Close off walls, to create a "room" with shape identical to your countertop.
- Use room-tool, double-click to create a room within the walls.
- Select the room, export selection to .obj format.
- Import the .obj file as furniture (adjust "countertop" thickness and elevation).
It requires some accurate measuring and tweaking to get perfect, but once you get the hang of it, it is not too hard.
Good luck. ok