Aim To determine the quantity of cotton underpadding needed for a comfortable, functional, below-elbow fibreglass cast. Methods In this randomised, prospective, crossover, clinical trial, 45 people with minimally displaced distal radial fractures were enrolled to randomly receive, 14 days after injury, a fibreglass below-elbow cast with either two or four layers of cotton underpadding. The characteristics of the recipients were recorded. After 2 weeks of immobilisation, participants completed a numerical survey evaluating parameters of comfort and underwent clinical and radiographic assessment. They then crossed over to receive the other fibreglass cast design, the process was repeated, and they indicated their cast preference, if any. Results Of 37 people who completed the study, 20 preferred the two-layer model, 13 the four-layer model and 4 had no preference. No significant trend for one design over the other was shown ( p = 0.1), nor any statistical association between preference and participant characteristics or the order in which cast models were received. Conclusions Participants did state a preference, but this was not related to recipient characteristics. Neither cast design was significantly the more comfortable; and both models were safe and functionally satisfactory.