I would get a 100W CHE and connect it to a thermometer. Set it to around 27-28 degrees. Set the thermometer on the coolest area of the enclosure at the height of the tortoise.
That way, it will turn off when temperature requirements are met, and turn on if temp goes down.
It might be hard to keep temperature even with that open design. Some members on the forum utilize "mini greenhouse" as a cover for the enclosure, and it has worked well with them.
I've just been and picked one up so will get that in for tonight. His temps stay pretty steady as the living room stays warm during the night, but by all means I will look at amending it so it's better. Thank you all for the advice. After reading about kidney failure in torts online I'm afraid to say I'm not feeling very optimistic. We have an appointment tomorrow at our local vet who will liase with the exotic vet who is far away and see if there is anything that can be done for squirt.