>>Today, my son found a young Eastern Box Turtle floating in our swimming pool. I have him/her right now in the land area, in a samll shallow bowl with water, in our Red Eared Slider tank. What is a low cost indoor house that I can put together for this little one?
First thing check the laws in your state, in some states it's not legal to keep an Eastern box for a pet.
I'm not sure I understand what you mean that it's in a small shallow bowl with water. Do you mean the turtle is sitting in water full time or do you mean the turtle is in the bowl and there is a separate bowl for it that has water in it?
Box turtles are basically land turtles that are more like tortoises, they live on land and then like to soak in water for only short periods of time. They can't be kept in water full time.
In general found box turtles are better off left in the wild but that always depends on what the situation is.
You can find a lot of info on how to set up housing for them through this link
Box Turtle
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PHRatz