Flat cap. Traditional.
I used to have same problem and I realised it was because I didn’t have a wind blocker fitted behind my head. I did have those style bars, that were like a a stretched M shape and made of stainless steel tubing.
I had a shape of 6mm glass cut to the required size with holes drilled in to be able to fasten it to the style bars with some special bolts and rubber washers etc. The glass had all its edges polished smooth and then the whole sheet of glass was toughened for safety.
Since fitting it I wear a light weight straw hat and I haven’t had it blown off even at 80mph. This was one of my best modifications that I made in twenty years of ownership of a Japanese version of the Mk 1.
If some one could send me the details of how to post photographs on the site I would post a few pictures of it.
Many years ago I posted lot of articles and pictures on this site but then the system we used started to cost subscribers money by the company that hosted them, so they were all lost or taken off, if my memory serves me correctly. I am getting on in years now so all IT stuff has to be explained to me in a simple way.
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A good covering of Factor 50 will do the job most of the time LOL 
and think only ever needed to raise the roof a couple of times when stuck in traffic due to it being too hot in nearly 25 years of MX5 motoring.
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