Ive been making my own liquid body for about 6 months and it doesn't include castil soap or putting bar soap in water. All the ingredients are available online and the end result comes out to a fraction of the price of store bought. It's also a great option for allergen control as all 7 of the ingredients are low reactives.
I'm currently pushing into shampoo and conditioner, both are a little more work than the body wash as they need to be heated and ph balanced. If my research is correct, all three can be made as concentrates for long term storage and all you have to do is add it to the water and xanthum gum before use.
If you're interested in getting into it, hit me up and I'll give you the deets. I have incredibly specific preferences in texture, smell, and experience so it's not perfect by my standards, but it's fucking amazing compared to buying it at the store and I've wanted root beer scented body wash since I was a child. Plus, you don't need anything special for the body wash, I mix it in a 4 cup measuring cup and put it in aldi Condament bottles.