There's no need for you to "reinvent the wheel" on building a range.
The NRA has a great book on range construction, range safety, and best practices.
I can't recall the exact name of the book. It might be as simple as the "NRA Range Manual" or some such.
I've browsed through the copy owned by my local private club and it talks in detail on what makes a safe backstop, construction techniques, safety fans, etc.
Contact the NRA and get a copy of the manual. I'm sure there's a charge, but I'm also sure it would be worth it. It is a bit "gun club" oriented, but you can just ignore the parts that aren't relevant to your situation.
Seriously, check out the resource that the NRA developed just to answer questions like yours before you go any further.