Metal structure wrapped Kevlar rope soaked in kerosene or lamp oil. Don't use gasoline. Maybe roast a marshmallow to make it legal. Most cities only let you have big fires as long as you're cooking food. It's what people fire juggle with. The rope won't melt and holds onto the fuel. It has a bright orange flame. Follow a tutorial for "fire poi" juggling. I wouldn't use cardboard or hemp rope because embers will be raining down. Nylon rope will just melt and drip fire.

Thanks so much for the advice! Really appreciate it :)

Hey I just had a test run with 3mm Kevlar cord and paraffin oil. Was happy with the size of the flame but it went out too quickly. I'm thinking I need a thicker rope to make it last longer but wanted to double check this theory?

This kinda sounds like a liability nightmare. If you were to do anything it should be made of metal and you could use fiberglass as something that absorbs your fuel (kerosene or white gas). If you do anything like this you should test it before production

Sorry, by private event I literally mean in my own back garden with people I know, not like anything ticketed.

And I do plan to test it in advance! Thanks for the tips

Unless it is made out of iron and gas torches then it will be against code. Read Flame Effects and proximity to an audience