My take on this problem. And a good exercise in designing threads.
Why buy travel-sized toothpaste at 10x the cost of a large tube when you can just refill the travel tube??
Just screw the big and small tubes together using this part and squeeze the big tube. After the refill, I blow the toothpaste remaining in the coupler part onto my toothbrush and brush my teeth, then I wipe out the insides with a Q-tip.
2019-05-24 - Under redesign to accommodate thread differences across manufacturer brands. Current model only fits Crest brand tubes.