how to create a spiral truncated pyramid by using plot 3 function?

how to create a spiral truncated pyramid by using plot3 function.

I want to crat a truncated pyramid with top part x=50 and y=50 and the spiral plot will decrease with a step depth of Z=0.5. and a total height will be -40. the angle of inclination of the pyramid will be 45 degrees.

thank you !