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Corona with a 5mm hook.
Or, a mix of plies to around 22 plies with a 6mm hook.
Genuinely doesn't matter. It needs to be a little stiff to hold it's shape. But around 4 stitches and 2 rows in 1 inch in half treble.
Mine measures just over three inches wide to 23.5 ish inches (but the knot takes out some length).
Chain 12 and working into the second chain from hook work htrs all along (11 stitches).
Every other row: ch1, htr all along.
Keep going for 48 ish rows (or until it measures 23.5 ish inches), checking that it wraps around your head and ears at the appropriate point and goes just past the point of touching itself at the top of your head.
Break yarn with a fair bit of tail.
With right sides together, wrong sides outside, fold the chain edge in half and do the same with the last row edge and insert them in together so that you have a layer of the first side, then the other side, then the first, then the second. Use the ends up sew all four layers together (this doesn't have to follow a particular method, just make sure you catch all four layers).
Sew your ends in, turn right side out, enjoy!