Off the top of my head one thing you could do is build a large enclosure with dividers so each section when divided is 8x16 which modern lumber comes in those dimensions so you could do it with out even cutting any parts and remove the dividers if the torts got along or have all the torts you...
I'd think he'd be hidden pretty well best chance would be when he's out in the sun looking for food and they may seem slow but can cover more ground than you'd think so give your self a wide area keep us posted
The bigger the box the more heat you will need to keep it warm I have a box 18" cubed with a 60w Che on a thermostat I also have a separate thermometer with probe in the box both sensors at the level of the top of my torts shell with my temp set to 80 degrees Fahrenheit and it holds nicely so...