Maybe have a second MHP?I agree, Blooie's heating pad cave is a great method, including outdoors in cold weather. How well do you think the OP's 25 chicks would do in that set-up? The 25 chicks is why I didn't mention it. I'm not talking about the dog crate, OP will use a shed which should be a lot bigger. The question is how many chicks can the heating pad cave handle, especially in a few weeks. after they grow.
Mine is 12" x 24", closer to 20" across top because I draped the heating pad down the sides.