I use a 40w RHP from Reptile Basics to heat the cool side of all my 4X2 enclosures and it works Great. It will not heat well past a divider the way the heat panel works is to actually heat the substrate and furniture in the enclosure which in turn heats the ambient air. I built my enclosures myself but lined them with Xpvc my room temps fluctuate from 65f to 80f and with the heat panels on a T-stat my ambient temps stay perfect. I would think with the divider you should be fine with a 28w RHP but I would give Rich at Reptile Basics a call he would be able to answer that for you and he does not mind doing so Hope this helps
Robie