If what you are searching for is complete sensorial deprivation experience, the only thing that can work is a sensorial deprivation tank.
You could do one in your bathtub but you would need something like 6-8 pounds of salt per gallon. Do the maths
The only affordable solution that comes to mind is to use only as much water as you need to hold up your whole body, reducing the total amount of salt needed by a significant chunk. This way it's also safer if you are alone and if no one can make sure that you don't go down.
You could try with less salt or using only warm water but it simply won't have the same effect and you won't float. Be careful in this case not to put enough water for your nose to go down.
The ideal is for you to lay down and have the front part of the face, the mouth and the nose stick out of water when you are completely laid down.
Once prepared the bathtub:
- wear blindfold
- wear earplugs/cotton balls wrapped in plastic/paper towel balls
Then enter your homemade tank, lay on your back and let it go, focusing on your breath only for 30-45 minutes
Tying ankles together might help with preventing the legs to touch the sides of the bath tub.