i'd try the clamp or u-mount that RAM-Mounts has to offer. They make good quality stuff

 

I've been using this Ram Mount for several years and its worked for me.

 

I remove the wind deflector and then use a strap mount to mount my Go Pro Session between the roll bar hoops on my 986. Works really well.

 

GoPro makes a mount that is meant for a bicycle handlebar. It has a removable insert so you can fit it to multiple size bars. Since it has the GoPro brand name on the packaging it's ridiculously expensive. It was an impulse buy at Walmart for me. I used it in my experimental airplane on the fat black cylinder of a stabilus type gas strut that holds the canopy open. I also have lots of RAM mounts including the one shown by 808croc. It is less tidy looking but has a bigger range of sizes that it will fit. There's a seller on ebay ("landloop") who specializes in RAM mount products in Clermont, FL. I've bought tons of RAM stuff from them and they nearly always have the best prices.

 

I used a stick-on mount on the plastic shelf between the roll bars on my 981. The stick-on mount can be removed by applying heat with a hairdryer and leaves the original surface adhesive-free. It is very secure. I've been using it about a year now. Those adhesive mounts work fine on paint as well.

 

I bought this cheap wrist-mount that works very well. It's double-velcro and, unlike most bicycle/roll-bar style mounts, actually fits over an OEM padded 987 Boxster roll bar. The clip is a difference size than GoPro OEM, but the screw mount matches the standard GoPro case so it's still really easy to use.

https://www.amazon.com/gp/product/B00MPEKBDU

 

So I think it sounds like most people are mounting it between the seats, my car has the bose system though and there's a speaker box there so I don't have that little bar to mount to. These suggestions look great but I'm not sure they'll fit around the headrest hoops though. I guess I could take the speaker out for autocross or track days though.