Thanks sounds like some good advice mate you have seen the cau tanks right?
CAU are by far my biggest inspiration.
I would suggest two ideas both of which i have seen done by CAU members.
Personally, I would not try the idea of creating a "fake" sloping base by using evaporation trays. Instead, try one of these methods.
1) V simple, have a layer of largish rocks towards the back of the tank and place substrate above them .You will be surprised how much room they take up and therefore reduce the need for as much Aquasoil.
2) Much more unusual. I think zig recommended it to me once, and then I recently saw it done on a CAU journal. Basically, using ladies tights, fill them up with substrate and use these "bags of substrate" at the back of the tank and place the AS above similar to the idea of rocks, just more easily molded into position. Now when I saw CAU do it, they used AS in the bags, i am 99% sure you could use any substrate, like cheap gravel. This has the advantage of just putting in the gravel straight underneath the Aquasoil since it will stop migration (i.e what we were talking about before, the gravel coming to the surface through the AS).
IMO, both these two ideas have an big advantage over your idea of a sloping make shift base, i.e like u said, using evap. trays, because these two methods will also make the slope more sturdy and stop it leveling out as much due to the natural water flow/circulation in the tank (topography).
Hope that helps.
Let me know what you decide to do.
Good luck
Chris