Mounting device for a flexible constructed retainer means for retention of a casting upper part in a concrete casting machine of the type which typically is used for the production of casting items in the form of concrete blocks for pavements and wall constructions. Typically, two or three fixer flutes (10) are applied for each casting upper part, the length of the fixer flutes are adapted the depth of the casting upper part. In the invention, a flexible mountable fixer flute (10) consisting of two identical halves (12) is described. When the height of these fixer flute-halves exceeds a certain height, spacers (14) are mounted by means of bolting. The design of the fixer flute (10) leaves a space where the forks of fork lift trucks can be passed through, thus using the fixer flutes to also lift the casting upper part.