Method for mapping & finding nest boxes

The National Dormouse Monitoring Programme (NDMP) involves siting a minimum of 50 wooden nest boxes in a grid throughout a woodland site. At out site we have 600 boxes and sometimes it can take hours to find them all. Some of the sections have been monitored for several decades so when management occurs or a tree falls down or a box needs replacing, its location is often slightly altered. In the past we’ve tried to GPS plot their location but it’s not very accurate. We would like to better map our boxes and have a way of finding them more easily. Does anyone have any advice please?