I was thinking of using of half of a clay garden pot as a frame? It would be more open at the front...
That would thwart the intended use of the MHP. You want the chicks to be able to access the heat from both above and below. Also, the "more open front" would not do as well holding in the heat and creating the dark environment that they are looking for. I'm guessing that the pot would also be too high in the middle, and not at all adjustable the way a wire frame is. Just my opinion, I'm wondering what Blooie might have to say about the pot.
I agree w lazy gardener. Even w the metal frame adjusted, I had to pile a lot of pine shavings to make it high enough for the chicks to touch their backs against the HP.