Shouldn't really need a strainer for modelling paints old boy. Those advertised are for use by decorators, or perhaps car body shops, where a 'quick' mix is required. Even then, it is in a way defeating the end result, as straining will leave some pigment in the strainer, which should have been mixed with the carrier. They are, to an extent, a 'lazy man's' way of stirring the paint without too much effort.
Modelling paints are made with much, much finer pigments, in a more refined carrier (hence, in part, the higher price per ml), and should be stirred thoroughly. A very fine mesh, like 'Sylon', or ladies tights, might catch some pigment, but to achieve the desired finish and shade, this pigment really needs to be separated and included with the rest of the strained paint, and stirred in.