OK, i think we need to come up with a sort of "non-magical item" type description for the shell. after all, if the sea hermit is killed, it makes an excellent (if not exactly portable) shelter. here are some bits we can use to write that up:
Surprisingly, the shell is indeed large enough, a majestic form spiraling upward into a small tower roughly 20 feet in diameter at the base and rising 30 feet high. The shell is usually white with deep blue and red markings.
"the shell is too thick and well armored to break through during the fast-paced action of combat."
"A rumor persists that a wizard has one of these great shells enchanted to act like Daern's instant fortress."
"This majestic structure is it squat, conical shell. It is approximately 20 feet across at its base (60 feet in circumference) and spirals upward 30 feet to a final room only 10 feet in diameter. In some areas, the overlapping spirals bend some of the ceilings, making the overall architecture lack the hard edges and corners to which most races are accustomed. In areas of sharp upward slope, the sea hermit has formed a polished stairway from the shell's glossy mother-of-pearl interior. The effect is quite beautiful.
The first 40 feet of the shell, as it wraps around the first curve, is where the sea hermit in its natural form fits. When posing as a humanoid, this portion is cluttered with driftwood and minor implements, suggesting a craft workshop or junk storage area. Past this, the shell has been reshaped to form archways, stairs, and rooms, including a laboratory and cells for prisoners. A few secret door, may tie installed, perhaps some ending in traps for overcurious guests; those often involve spring-loaded tridents tipped with the poison of an exotic fish (THAC0 11, inflicting 1d10+4d8 points of damage; a successful saving throw vs. poison reduces damage to 1d10+2d8 points)."
