wobbly single wiper?

I know lots of people have done single wiper mods to their Mk1's but how should the wiper arm/spindle be secured to the panel?
My golf had a single wiper fitted when I bought it so I don't know what kit was used or if it's a DIY job, it's a centred one if that helps. Basically the spindle which the wiper arm fits too keeps coming loose and moving (it's now starting to damage the paint so I need to get this sorted properly). Any ideas?

cheers
Mat