I have several hovabaters and they work great. I never thought about adding a helix though. Might be a good idea. The problem with the hovabater is all the Off/On Off/On Off/On stuff. If you use lots of hatching medium it will hold a more constant temp. Also if you have room toss in a big brick to give it some density which will also hold a more constant temp.
Big apple sells a new temp controll device like the helix for a little over 30 bucks. Im going to buy a couple of them for my big incubator im working on.