I actually managed to stretch 1/2' ID cristallo over my 19 mm pump intake...It didn't look too pretty so I changed it to 16 mm ID ordinary (shitty) stuff, just for the part from flow indicator to pump intake...
Cristallo is good, but a little too much on the memory effect side for me...but it's dirt cheap here (1/2 euro per m for 1/2 ID tube)...
And, I heated mine over the gas stove, after I measured and cutted the right length...