At the moment we're heading south down the east coast of Australia (from Cairns to Sydney ) and damn the sun is relentless...
Anyway, since I really like my funky new bike helmet but it provides no shade for my face or neck, I've adapted an eleven dollar white canvas cricket hat to the task.
Step 1: Necessities etc -
sewing machine / #10 hand sewing needle
pencil / chalk
Step 2: The Process -
Find a suitable candidate. White to reflect the sun, canvas to breathe, and CHEAP.
Cut the crown off - to minimise discomfort and maximise ventilation. I cut less off the back to cover the big gap. The front is about 30mm high, and the back is roughly 80mm...
Mark the slots for the chin-strap, and cut out with the scalpel. It's easy to cut more out later, but harder to fix if you remove too much too early... I cut the slots about 4mm wide and 120mm long
Check that it all fits before you finish it...
Bind the edges. With a sewing machine, or by hand, zig-zag or blanket-stitch over the cut edges to keep the fabric from fraying. Reinforce the sliced-through vertical seams by sewing back-and-forth a bit.
Now whack it on ya noggin and get cyclin', safe from the sun.