I didn't want any bolts showing, so I used bolts with a flat head and countersunk them into the metal, but they still show through.
The other problem is I didn't quite drill the holes in the right place (stupid me!), so they have to be able to move in order to put the sticks on.
In the future I will use threaded rods (bolts without heads), and just epoxy them to the bottom of the panel.
I suspect there is little hope in concealing the current bolts, especially since there is no way of removing the vinyl without destroying it. Oh well.
Anyone care to give an opinoin on what colour buttons to use?