You could also go for a longer L shape (further in to the sun) and just make it removable so when its not in use it can be out of the way. The only issue i see with it is going to be ambient air temp. Im not too geared up on torts atm but im a keeper of snakes and know a reasonable amount about reptiles in general and ive come across similar issues in the past. Both torts and reptiles are cold blooded so exactly the same rules apply. A torts basking surface temp will be different to the air temp around it and we all know what its like walking in to a greanhouse on a baking hot summers day, sometime you struggle to breath due to lack of ventilation not to mention the high, sticky humidity. If youve got a plexi glass container in the sun warming a hotspot to 32 ish without ventilation then the ambient air temp will also be as high as 32. You will need vents in the bottom and vents in the top on the opposite side. As the hot air escapes through the top holes (as heat rises as we all know) it will suck cool air in through the lower holes. Just something to think about as it will definately cause an issue on a hot day. The tort can go back outside to get air but then he cant get warm at the same time.That's exactly what I was thinking. I thought I could cut a section in the short side of the base big enough to fit the entrance of the coldframe. Then I could prop it up on bricks, leaving a gap so that Pip can come and go as she pleases, if that makes sense.
I got inspiration from this picture, especially how the cold frame is elevated instead of having a door cut into it: