My idea of crafting recipe is that you take 5 of whatever Gem is associated with the type, and put them in a crafting table in the same arrangement as if you were crafting a helmet. I felt this makes sense because Terastallization makes the Pokemon have a gemstone-like appearance to it and the Tera Crowns themselves look like they are made of some type of gem or colored glass.
The reasons why I want this feature is that whenever I see some of the Tera Hats such as the Ghost-type one, I wish that I could wear them myself on some kind of crazy hat day, as several of them look kinda cool. Also, I imagine (or hope) the Tera Hat models would get added when Terastallization gets added, so you'd already have the models and would just need to give those same models to an item that can be worn by players or placed on the ground. Also, even if you aren't looking for them, you can end up with quite a few Gems if you open loot chests or participate in raids. I usually just end up selling them since I don't need them for battle, so this would give an additional use.