I know this thread is quite old now but thought I would add my experience. I have just completed this mod (my first mod on the new car) and wanted to share what I used and where I got it from, hopefully some current links to parts will help.
I used the following gas struts (as recommended in this thread):
2 x Febi Bilstein 22709
https://www.ebay.co.uk/itm/2x-Rear-Tailgate-Gas-Support-Strut-for-Seat-Ibiza-FEBI-BILSTEIN-27709/183281278894?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
And the following 10mm ball studs:
2 x M6
https://www.amazon.co.uk/gp/product/B07LFZZ96F/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1
2 x M8
https://www.amazon.co.uk/gp/product/B07LG92X6B/ref=ppx_yo_dt_b_asin_image_o03_s00?ie=UTF8&psc=1
and I also used a washer on each ball stud. These I had already but M6 and M8 washers will be suitable. Lastly I added a bit of ThreadLock to each of the new ball studs.
It has come out perfectly and really pleasedwith the ease of opening and closing the bonnet now. thanks to all who have given advice and guidance.