I was going to design a whole shelf to help the bottle stand upside down without falling over, and then I realized all i need was a cap to slip over it. Now my shampoo and conditioner bottles are able to sit upside down I can get every last drop.

I printed this with Overture Clear PETG, and I left a hole in the cup so water can drain out and wont collect inside.

I buy the large size bottles on ebay to save money, but I think this will work for smaller bottles as well.