ideas needed for
 

i have a 8x8x8 nano cube. (okay my daughter 'designed it' and i maintain it) she made a 'river' of glass stones and wants a barrier wall so the 'main' part of the tank is higher and has plants.

she does not want it sloped

any ideas on how to build a 'nano retaining wall' (i know aquarium sealant and XXX) but what would be the right size, how do i find stones that size? will i need "arms' to hold the wall in place (like with real retaining walls) or since it's just 8 x 2" tall can it just 'stand' if curved?

this one has me stumped