Ive done something like this with my (current) first tank, and can suggest some tips, though as I've learned more after putting it in place, I worry I've stored up problems for later.
I used a mix of banking techniques: Epo Putty to stick the big bits together and stabilise them against slippage; a small layer of pea gravel in between round the base, then aquatic soil, topped with fine gravel. Very small pieces of slate or acrylic sheets (found via a model railway supplier) can also be tucked in to further shore up fine substrate slippage.
The worry: I now have a very deep section of substrate, which can risk build up of bacteria that cause hydrogen sulfate pockets, which can be lethal to livestock if the pop. The pea gravel reduces the risk a bit. Ensuring no deep rooted plants go near that area is another thing to avoid (in case you ever have to remove them) . Also avoid any fish that might be enthusiastic diggers, and generally try not to disturb it.
I'd not do this again, to be honest, until I've learned a bit more about working with deep substrates.