My idea was to take 2 toothbrushes, measure out the width and length of the bottom of the brushes with my trusty digital caliper, then make a long attachment so I can reach deep down without using my hand. I used pre-made shapes in tinkercad.
My first attempt was a dud. It didn't fit inside the bottle! My second attempt with the entry holes very far down offered no support to the brushes and they just went willy nilly everywhere once I got inside the bottle. Now granted I could have taped them together or used elastics but this was not the point.
And tadah! My one concern is I didn't account for trying to clean the very bottom. That will be a challenge for next time.