I think there’s about 20 of them. He’s suggested it and they aren’t interested because the units are secure from breakins but the site isn’t secure from travellers. It’ll be if travellers turned up the tenants would then complain.How many units?
CCTV and 24/7 security guard on a gate and a fully fenced perimeter divide the cost by the number of units and charge them for it.
Tenants should pay more for a secure site.
If it's only smaller parcels and not pallets, could you install a locker system similar to what you see on garage forecourts in towns. Then couriers could leave parcels in a customers locker