Through trial and error I have determined that the threads are 7mm, I can find that size in zinc, but not in stainless.

The bolts come through the top of flange of the light casing and are exposed above.

Is there something I can cover them with to protect them from the elements ?

Should I just tape them up real well w/blk electrical tape ?

Where can I find 7mm stainless ?

Thanks to everyone that contributed.