ebay seller QualityPorscheParts has the spinlocks you need including the retaining bolts for sale for $37.50 US:
https://www.ebay.com/itm/00-Boxster-S-Porsche-986-L-R-CONVERTIBLE-MOUNTING-BRACKETS-98656105300-37-778/282618318954?hash=item41cd5d506a:g:PQMAAOSwdtlZmdJ F
You'll remove the spinlocks and retaining screws, and install them in your Boxster. Then throw away the brackets (or save them as momentos).
The listing says that they don't ship to the UK, but perhaps if you call them you can work something out.
You'll need a long handled 5mm hex screwdriver to install the retaining screws.
Someone who has done it before can probably install them in about 15 minutes. First timers will probably take closer to an hour.