We also use plastic storage boxes (from poundland or Asda or something like that) with a square hole cut in the side and an Osb board on top at an angle, so they don't lay on there (and it is underneath the perch).
Have to say, never had a problem with mites since we have a system of bi-annual creosoting..., but still prefer to be able to sterilise/properly clean the nest boxes once in a while and after incubation period.