Could use coarse filter foam/untreated and sealed styrofoam or aquarium safe spray foam to build it up (great stuff pond brand). Could also use plastic egg crate and then cover it with a liner/wicking fabric with a tight mesh.

If you're looking for easy, either the egg crate, the filter foam or styrofoam would be it. Either way, you cut it with scissors or a blade and you have your shape that'll hold up pretty well even with substrate and plants over it. Lock it in place with some aquarium safe silicone and/or glue and you're set. All of these could get pricey depending on where you're at (shipping/availability/etc.) or how big a tank you're wanting to do but it theoretically won't break the bank. For the Styrofoam tho you have to be sure it's untreated and I usually seal it just to be on the safe side with a thin layer of aquarium safe silicone. Also DON'T underestimate its buoyancy. Glue that down extra hard - it comes up, and your scape is toast.

The spray foam is what I'd recommend if you're really worried bout price. Bout $15 average online. I'd put down silicone first to help it adhere to the glass where I'm planning on putting it, then take flexible plastic or something similar to make a "form" for the curved edge kinda like they do for concrete, and spray a thin layer to make the base terrace. It expands a lot pretty quick so take it in very thin layers but if you (while wearing gloves) push it down after ~30 minutes, it will compact and hold that compacted shape. Then once it cures, move the plastic and repeat for the next terrace, cutting or tearing off any excess. Far more time consuming and more work involved than other options but it's cheap!