That's funny, because I've always imagined that medieval underwear was really lousy. The materials available were wool and coarse linen after all, at least in Europe until cotton became available in the late renaissance.
Mine is in linen (well, not
exactly medieval linen but we were very careful in the choice of fabrics) and adjusted, but impossible to find a picture
When I was in home after a reconstruction I often stayed with this clothing after a fight to remove the heavy dress.