There are a couple of companies that I have found, First Walltech and Graditel (videos on youtube), that sell permanent concrete forms. The forms come in various widths, 3", 4", 5" and 6". I would describe them as two 4'x8' pieces of hardi panel with spacers sandwiched in between. Very little finishing required. The suppliers are on Luzon but will ship. Maybe you could get a store to special order or just deal direct if not.