Welcome! In planning a con costume for 2024 I planned being the Black Knight, and my dad volunteered to be King Arthur. I found great versions of the Black Knight helmet, but virtually nothing for King Arthur. After much combing of the desert I found the basis of the crown is a 13th century Kettle Hat, so I used one of those to establish the proportions of the center piece, then movie reference of the Fleur-de-lis to model the border. Though an avid hobbyist in Photoshop, it was really my first bout with blender. I learned a lot. I hope it's helpful to many cosplayers and fans of Monty Python!
The crown is modeled for a Large head, approximatly 7¼ - 7⅜ US or 58 - 59 Metric hat size, with room for some padding and/or chainmail. I recommend using a head model and/or cutting a small section of the bottom to print first to test sizing before commiting to a bigger print.
There's a full crown model, though printing it on my printer wanted a great deal of support material. To limit this I cut it at the top since plenty of post processing was already planned. Similarly with the top piece wanted lots of infill support, so I cut it again and printed the two pieces unsupported standing up. This will likely depend a lot on your printer, so make your best judgement based on what you know, and what your needs are.
Dimensions:X 242.01 mmY 145.43 mmZ 275.67 mm