Way to small for outdoor pool or in ground pond. I would not. Unless you sit there watching all the time. Raccoons are by me to in city limits. Ones by my house I see are around me at 2am - 5am like clock work. Heron's also here , a baby turtle would not stand a chance. Snappers bask in shallow water for sun and a baby will be a target. Maybe when he gets about 3#'s I say outside would be ok. I only had one snapper in my time was about 14#'s until Hurricane Katrina got her and several other turtles. Never wanted a snapper but I took it away from a so called friend that was going to cook it. Turned out to be a great turtle.
Take your time , learn all you can . When you get everything down to a science then think about more. It's not cheap to setup a turtle at first , lots to learn too , for a happy and healthy turtle ! Save up ! I made a few ponds for neighbors and a small 5'x9'x2' deep pond is very small but cost $$$ to do it correct.
If just a simple area temporary to get sun with you watching him (a good fence with a kiddie pool) can work just remember you have a snapper they don't dry bask they "water bask".
For an idea I use this design for ponds and kits you can get. Video #1 is an idea for a good pond small but good for goldfish , Koi or turtles with a little modifying for safety and type of turtle! Instead of rocks on bottom , for a snapper I would go with mud on the liner. Also a fence and any cover may be needed.
https://www.pondgardener.com/pages/how-to-videosAnybody ever tell you that a common snappers get's real big and up to 30 lb's ? That depend on it's DNA could get bigger.