Silicone Bottle Topper
Whether using your liquid sensor tube or not, these caps will keep the bugs out! They'll also keep out dust, and slow down evaporative losses.
They come in 4 sizes:
Standard, Large, Extra Large, and Jumbo.
Standard will fit most 750mL bottles out there.
Large is good for most 1.5L bottles.
Extra Large is for glass milk bottles, cranberry juice, etc.
Jumbo is for extra wide mouth orange juice, etc.
Check out the evolution of the design here.