Amorphous selenium (a-Se) possesses unique features that have been leveraged for medical imaging applications. Previously, it was shown that having a soft interface with a-Se will reduce the stress generated from the creation of crystalline nucleus, prevent radiation-induced crystallization, and improve long-term stability of the device. The aim of this study is to develop a uniform thickness of a-Se on a flexible substrate, polyethylene terephthalate coated with indium tin oxide (ITO-PET), for future investigations of the effect of substate on stability of the a-Se against radiation-induced defects and long-term storage. In order to fabricate the a-Se-based detector, we have developed a dedicated thermal evaporator for a-Se deposition. The dependence of the source-to-substrate distance on film uniformity has been investigated. Experimentally, following modeling results, a-Se samples over a 2-inch diameter were deposited on ITOPET and thickness uniformity was compared to simulation.