I want to fix a trellis to our fence so I can grow clematis up the back of it. We have large fence panels with concrete posts in between them, drilling the concrete to put in plastic fixings for the trellis appears to be too hard for the drill bits we have, and the fence panels themselves aren't that thick so drilling into them seems a bit like it might not hold. Can anyone suggest the best way to fix the trellis? Or should I find a different way to train my clematis?