Whatd i'd do optionwise
1st (best) - Get some 'shelf' LR and make a dish out of it (so it'll hold sand). Find a nice spot for your bowl + sand higher up in the tank (maybe 5-10" off the surface). Put the clam in it! The higher up you go the better (within reason). I'd say if youre willing to put the clam up within 5" off the surface you can handle ANY clam. <--Now i wait for chrisandbarb to yell at me :p
2nd - Use some tupperware to form your little "bowl" and fill with sand. Repeat above ^^ with clam.
If im not mistaken the lighting hierarchy goes as follows (usually - depends on exact animals)
Maxima - Most intense
Crocea
Dersa
Gigas
Squamosa - Still intense :p