To answer your question, I also used a sand substrate and couldn't figure out how to keep it sloped. I visited my local hardware store to price out PVC or vinyl as it is fish safe and inexpensive. I recently learned from experience that it is less expensive than sand substrate and offers the capability of adding height to the aquascape without the weight of substrate or the extreme buoyancy of polystyrene (foam). PVC and vinyl both are just barely buoyant enough to float.
PVC pipes are no more than $5 for a 10' pipe (you will want varying diameters of these)
Unpainted vinyl gutters are about $8 for a 10' section
Unpainted vinyl gutters modified can help if you shape a sort of "stair case".
PVC piping also functions in the same manner with less of a dramatic edge. Using PVC piping of varying diameters, you can fashion a type of "slope" by putting the larger diametered pipes on the side of the highest intended part of the slope and working your way to the lowest end of the slope with progressively smaller diametered PVC pipes.
A lesson I learned regarding "keeping substrate on the structure" was to add a pipe of slightly larger diameter as the last pipe intended to go at the lowest part of the slope. This is what was going to help me keep my substrate on the slope from sliding down. i.e. if the last pipe (at the lowest part) of my slope is 1", add another pipe of 1.5" or 2" diameter next to it to function as a sort of "lip".
Don't forget to cap your pipes so you aren't losing lots of your substrate inside the pipes.
I just finished a project with this build, and while I used lava rock as my substrate (for other reasons), sand can be used just the same.
55g, brackish w/2 puffers, bumblebee gobies
Sand substrate w/java fern, java moss, red mangroves
Drilled and custom plumbed 29g sump
Odyssea quad T5HO
High tech CO2