I used to rehab orphaned baby squirrels, and this is the incubator my DH built for me. I hope it might help.
How to make an Incubator
It is the first incubator mentioned on that link. DH built it from materials we had on hand, and it works great for infant squirrels. I have used it with numerous baby squirrels. I don't know how well it would work for a puppy, and I don't have time right now to check out what puppy incubators are supposed to look like. This incubator works very well for baby squirrels though, as long as they are young enough to still need to be stimulated to potty. Once they start pottying on their own, you must move them into other containment. But this worked very well for me for the first couple weeks of their lives. I have raised babies from birth in this type incubator, and newborn squirrels are so tiny, 2-3 of them can fit in a film canister. I would think this incubator would work well for yorkies too, as long as you are still stimulating them to potty. Once they start to pee on their own, you must get them out of that warm plastic box! I think it would work well to get you out of the critical stage though.