What part or kind of geoduck gives crunchy slender slices?
I suspect you cut your geoduck “the wrong way” (to get the specific shape, that is). If you want crunch, you should use the syphon, so that was a hit already. For the wavy slices, I suggest watching the first recipe in this video by chef Hung Huynh. In short, he


uses live geoduck 
quickly blanches the neck by immersing it in hot water
peels the outer skin off and cuts the neck from the body. Then he
slices the neck lengthwise and washes it in cold saltwater.


When he thinly slices the halved necks at a bias, he gets longish strips that curl “due to the contractions of the muscles”. 

While you can still cut your frozen geoduck neck in thin slices, I doubt that they would curl the same way fresh do.