Right now the ‘fish eye’ tool is a glorified ellipse tool, it can quite literally only draw ellipses. You can draw on one axis out of 3. You need to use a total of 3 different perspective rulers (2 fish eye and 1 vanishing point) to draw a box in fish-eye perspective. It would be nice to be able to draw a simple fish-eye box with a single fish-eye ruler.
Here is a fisheye box I drew using one fisheye ruler