With the description above :)
more detailed here
Take the bottom Side line of your Surf
maybe you must join its different part for have a unic curve
Rebuild it for have less points
extrude the "any curves lines" who trimed your Surf
Move the bottom curve line for be smaller
move some points to the extremities of the "any curves lines"
Extrude it for have a perpendicular plane
and make some Boolean diff of all that :)



Here view 3D