Giant clams were at one time scurrilously portrayed as capable of trapping divers underwater until they drowned, and the myth persists despite evidence to the contrary.
I have been searching for giant clam miniatures for a long time because I wanted to represent the Arduin monster known as the Mantrap (aka Leg Eater, Jaws, Big Mouth), which was essentially a giant land clam.
It probably seems strange that I would be interested in such an obscure monster, but I bought my first Arduin book when the film Dark Crystal was released, and when I saw the sequence at 0:27 in the following clip, it crystallized the image of the Mantrap in my mind.
The thought of fighting giant clams in RPGs seems rather silly (ala the giant mutant fire clam from World of Synnibar), but I was surprised to find that monster clams did appear in real world mythology.
In Chinese mythology, there is the shèn/chèn (蜃), a giant clam (or sometimes dragon-like creature) that released bubbles or mist from its siphons that rose to the surface of the ocean and appeared as fabulous structures or landscapes to observers.
The ideograph for shèn is incorporated into the Japanese (and Korean) word for "mirage" (蜃気楼; shinkirō, shèn breath tower). The shèn also makes an appearance in the series Naruto as the ōhamaguri.
A giant clam is also found in the Tahitian myth of Rata and its derivatives. Pahua-tu-tahi (Coral Rock Standing Alone) was a sea god in the form of a giant clam the size of a mountain. It attacked by rising from the depths and opening its valves so that its victims were drawn in with the rushing water that filled its mouth. Then it would close its shell to entrap them for leisurely digestion.
In some legends, Pahua-tu-tahi has been transcribed as just Paua. It was also known as one of the Children of Puna.
For giant monsters like Pahua-tu-tahi, there are lots of aquarium decorations or even real clam shells that can probably be used to represent it.
Normal sized 1/72 scale giant clams are more difficult to find. It is only recently that I've discovered some nice giant clam models from Morland Studios in their Sea Accessory Set 2. The set comes with one sprue consisting of two giant clams, and another sprue with four small clams.
The giant clams work well for moderately large giant clams, but the small clams are oversized, and probably better suited for 1/35 or 1/32 scale.
While the Morland clams are very nice, options for the largest specimens of giant clam remain elusive. I did stumble across a toy that was part of the Matchbox Mega Rig Shark Adventure playset, but it scales out to 10.5' wide, which is about twice the width of a large giant clam.
Unfortunately, the Matchbox clam is toy-like, with hinges and a socket underneath that detract from its appearance. It also has poor shape when viewed from overhead and the folds of the shell are not well defined. Still, with a little work, I believe it could make a passable giant clam.