The meat is already cooked, and just needs reheated, but the pastry topping is just dough, and that needs dry heat to make it rise up and crisp.
Low heat doesn't do that well.
You need to take the lid off entirely to let the pastry rise....that's why it's an 'oven' pie.
However, with care, you can bodge an oven from a bigger pot and lid. Just make sure you rise the bottom of the pie tin off the bottom by resting in on either a layer of clean sand, or a few little stones, and don't jam the lid on tight or it'll just steam the pastry.
Honestly ? if you haven't tried one in a while, I think you might best have a shot of one cooked at home first. They're pretty cheap round here. £1.49 last I saw them, I bought Himself one. He said it was fine, just smaller than he minded. I'm pretty sure folks used to use one to serve four, but well, I know we didn't eat such big servings, just had more courses, when I was little, but I couldn't see this pie serving four