The smallest width of that groove is maybe 3/8", no way a 3/4" tube would fit in there. This is the closest thing McMaster Carr has that might fit.
Flexible High-Pressure Nylon Tubing, Semi-Clear, 0.225" ID, 3/8" OD $9.85/5 feet + shipping.
Not questioning the importance of this thing, but if it's so important, you'd think Sunfish would have it available in the 6" that I need for a buck...guess I'll have to start visiting hardware stores.