Some time ago I made the flail snail on the right; more recently I made a second, more naturalistic flail snail.
The new one, on the left, has a real seashell for its shell (lightly washed with dark brown), and the body is made of polymer clay. The flails were also polymer clay, formed around some florist wire so they’d be less likely to break, and to give them an easy way to attach. The older one, on the right, has a shell from a cheap plastic animal, a body of epoxy putty, and the flails are wire with mace-heads from cheap plastic knight toys.
While I have no doubt that others have made their own flail snail minis (this lovely one came up early in a GIS), I do find it odd that no miniatures company ever made them. They hardly require much skill, though I guess as one of the infamously “dumbest monsters of D&D” lists, and an example of what’s so terrible about the Fiend Folio, the poor flail snail is subject to too much ridicule to get a fair break.