I'd like to fit trellis (3ft) on top of an existing 3ft wooden fence. Trouble is that the fence posts are only slightly higher than the fence so I figured I'd probably need to find a way of extending them so that they can support the trellis. I saw some metpost fence post extenders in B&Q - are these any good? Alternatively can I get away with getting 6ft trellis and attaching it directly to the 3ft fence and not bothering to extend the fence posts at all?