Just get 20 litres of head shrinking fluid from Screwfix, they should shrink to 10" in about 20 minutes. Best to get them removed from the body first though. You can use expanding foam to fill any small gaps left. There are some other options from boutique manufacturers, grotesque features, ethnic variations etc. B&Q do some decent faux piercings if you want to go full native and they have a discount card if you shrink a lot of heads.
Just get 20 litres of head shrinking fluid from Screwfix, they should shrink to 10" in about 20 minutes. Best to get them removed from the body first though. You can use expanding foam to fill any small gaps left. There are some other options from boutique manufacturers, grotesque features, ethnic variations etc. B&Q do some decent faux piercings if you want to go full native and they have a discount card if you shrink a lot of heads.