Maybe something like this...
-Blend the extremities
- Join all
- Vertically extrude
- Kill curves for have only edges
- Nsided with the bulge that you want (can be also null or negative ! )
- Trim (if wanted) the extremities for have the 3 original start curves as perimeter
(not shown on the animation)

If you make the same for the bottom and join all surfaces you will obtain a "solid" !

Ps It's not a good thing to put out the space your objects!
(calculate errors can appear)
Better to put them above the grid! ;)