Comfort is one of the most relevant issues in the shoe industry since there is an increasing demand for improved comfort in detriment for aesthetics, and users are becoming more and more aware about its beneficial effects on health and wellbeing. However, comfort is a difficult feature to define since it is a subjective concept, which perception depends on the user, and therefore difficult to model on an engineering level. However, there are some parameters that can be related to the perceived comfort. Several previous studies related the increase of plantar pressure to pain and discomfort but up to now there is no clear idea of what is the pressure distribution that most favors comfort perception. Having this in mind and the geometric freedom provided by 3D printing, this work aimed at exploring the relationship between the insole structure and comfort. For this sake, height heel zone inserts were designed and produced, by Fused Filament Fabrication (FFF) with a TPE filament, for later insole insertion and test by several users. The inserts have different internal structures, i.e., they are hollow and have different filling density gradients automatically generated by a devoted in-house routine. Pairs of the different interchangeable inserts were used in slippers whose midsole has been laser cut to accommodate them. The users tested the eight types of insoles and answer to an inquiry targeting to characterize their perceived comfort. The results obtained, associated to the well-known typical plantar pressure distribution at the heel zone, enabled to find out a relationship between local pressure/hardness-comfort, providing guidelines for heel zone (structural) design. Still today, designing a comfortable shoe is not a straightforward process. However, 3D printing technologies as FFF, due to its vast filament materials, can reach new levels of comfort and usability, and more important, in a personalized manner.