Nice job with the stand, it looks great.
I'm really looking forward to seeing the new set up.
There are some good techniques on dendroboard for making walls and such.
If I remember correctly, the way most recommended is such:
Use Great Stuff expanding foam to create the hill/wall, and embed large elements with the foam still wet.
Let the foam cure, and then do any shaping necessary.
Once you have the hill the way you want it, coat all the foam with a thick layer of black silicone, and embed a concealing top layer into the silicone.
I completely took it apart because it was not square. I reused the 2x4s and added a few more. Then all new plywood. I am not 100% happy with it. But without a table saw or chop saw, it was about the best I could do.
I am only going to fill with about 90 gallons. I used to have a 125, then a 90 before after talking with my uncle who used to build houses. I don't think 150 would be a good idea though.
It is against an exterior wall and crosses 3 or 4 floor joists. I am not worried with the set up I am planning.