The only thing I can think of would be to put some re-inforced sheet steel(like what they use on petrol garage forecourt floors, sunk into concrete for added strength) and place these on the walls.
secure to the walls somehow and then render over the top of the mesh, this should be more that suitable for what you need, then you could add some insulation before putting in pond liner.
if your building a pond that big, why don't you keep if exclusively for koi anyway ?
here's a link to what has been done by a professional pond builder...see pic 7 and 8 in the link.