If we assume that there will never be a TH11, and there will never be L12 walls, then we have plenty of options for fixing the elixir walls. My preferred solution is actually just to give the L9 walls to TH8's. The next best solution (imho) is to make L9 walls gold-only again (regardless of whether we give them to TH8). Goobers' suggestion (or a similar variation) is my next best option. Finally, my next best option is to make elixir walls more expensive than gold walls (not a reserve, but actually increase the price). Either tack a flat 1M on the price of walls purchased with elixir, or come up with some kind of sliding scale.
But I like to assume that there will be a TH11 and L12 walls some day. And when that day comes, Goobers' idea is the one that survives it best, without any modifications. Whatever the new lootable max is at TH11, that would be the reserve. Let's say 8% limit and 500k cap means 6.25M max, and let's assume they make L12 storages with 3M for a total of 12M. Then L12 walls at 5M a pop would require 11.25M in storage.
BTW, I suppose the best option is for Supercell to give high level players a true end-game for elixir, much like walls are an end-game for gold.

