For that size tank, would think a Heat Cable connected to controller and long enough to do at least two passes over the tank bottom outside (now vertical's back) would spread heat over a larger area. Now if building a background that position would not work and would have to affix to a non covered side.

Another thing to consider is to build a frog proof cage and instal in it a PC cooling fan. That will help to move air around inside enclosure and avoid a too large temp gradient between top and bottom areas that are close to 4 feet away .