this is what i would do:
1. get a tub about the size of the pool you want
2. drill a hole in the side about 1", you will be inserting a pvc pipe into the hole. insert the pipe at the depth you want the water level to stop. (1" is the inside diameter you may need to file the hole to fit)
3. get a 90 degree fitting and go through the enclosure floor with the pipe down into a sump bucket.
4.put a filter in the sump bucket and have as much hose as you will need to reach the top of the waterfall. you can drill a hole in the floor or in the back of the enclosure and route the return hose as you wish
5. silicone around the pipe in the tub and on the floor.
6.fill in the areas around the tub w/ foam making it look as you desire.
7.after the foam is dried (about 24 hours) lay the enclosure on its back and lay the filter return line out where it will be
8.get a tupperware the size you want your top reservoir to be and tape it to the bottom (which is the back when upright) at a 90 degree angle
10.make the waterfall part now with the foam, spray it directly on the back of the enclosure make it the thickness and shape you want. the top tupperware should be completely surrounded(but not inside) and the waterfall portion should connect to the pool portion at the bottom. the filter return line should be in the foam so it stays when it hardens. the open end of the hose should be angled into the top tupperware reservoir.
11.let it all dry and paint as you wish , i like the stone fleck paint on the foam, it looks pretty nice. after the paint is dry then seal it with the marine epoxy. any problems with the toxicity of the paint should be ok with the epoxy sealing it, but just in case you can opt to not paint the areas that will have standing water.
i may have missed something, i am kinda hashing this out right before dinner, so im in a hurry, just ask if i am not clear on something.
also, waterfalls are notorious for splashing, leaking and just plain getting things wet. it is best if the enclosure can handle water or is water tight.

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