Hi, thanks for the question, and sorry for taking ages to reply!
The crowns/hairpieces men wear to secure the buns on the top of their heads are called guan/冠. There’s a Wikipedia article on guan here. The smaller ones that cover just the bun are called xiaoguan/小冠 (small guan).
I wrote about guan and xiaoguan in my posts here: 1, 2, 3, 4, and I also have a xiaoguan tag here. Please check them out!
Hope this helps!
(Image is from cdrama Love Between Fairy and Devil)













