I've painted the windscreen frame on my Westfield black (was aluminium colour), so I now need to replace the snap fastener posts on the frame for black ones, for the hood to clip to.

I thought the ones I had were 15mm, as the post on the screen frame measures 10mm diameter.

A quick look on eBay and I found some that seemed the right size, but they are actually just too small (by about 1/2mm).

Any ideas what size I might need?