hi i'm trying to build a one piece concrete box fountain and what i mean by box is one box on top of the other box,so i want to cast it in one piece but dont know how to build the forms to do that nor do i know how to do the floor of the box plz help

I'm not entirely sure I fully understand the question, what is the application of this?

I can see the box in the first image, the second looks like half a pyramid.


If this were a house or shed foundation, it may well look like the first image.

You would pour that in 2 or 3 phases, the outside footers would be poured first (but going down underneath the slab 2' or more).

Then you would pour the slab right on the gravel floor after the forms are removed. This is often done with the rebar positioned prior, as not to have to tie anything in between the 2 pours.

Something like this. (https://www.thebarnyardstore.com/upload/images/foundation/1500/Concrete_Foundation_Detail2-0.jpg)
https://www.thebarnyardstore.com/upload/images/foundation/1500/Concrete_Foundation_Detail2-0.jpg

If you're just trying to make a box for another application, you would form something like this:
https://diy.sndimg.com/content/dam/images/diy/fullset/2009/9/14/0/dycr210_fire-feature-interior-form2_s4x3.jpg.rend.hgtvcom.1280.960.suffix/1420796177118.jpeg

Except, lowering the center part of the form, maybe by one run of 2x4, and then placing a piece of plywood over the center hole. It would have to be poured upside-down, then flipped after curing.