SO! Im sooo stressed out!

I guy brought 3 red tails in to the animal hospital that I work at. All three were so emaciated and dehydrated. 2 had to be humanely euthanized because there condition was so poor. The other one I ended up taking from him because I could not see it go back to those condition.

So now I have a malnourished and dehydrated boa. He/she is active and took a medium sized rat for me but I honestly dont have him in a big enough enclosure right now. I was wondering if anyone has ever build a tank and if they had directions on how to do it. Im a pretty handy guy and I feel like that would be my "cheapest" option for housing him.

Also, what size tank should he be in? Should I just guy one?

Like I said, his tank is small but not "cruel" small, i just know that it could be bigger.

Thank you all in advance for the kind words and help!