Try s-5 standing seam clips – they are not exactly made for what you intend to use them for, but will work. It’s strange that the snow won’t slide off the roof, but most likely it is because of the lack of sun light.

You may also try snow guards – they will keep the snow / ice from sliding off, until it melts. s-5 clips will do the same thing, but snow guards won’t need the use of electricity to run the heating cables all winter long.

Good luck.