Use an inverted "J" channel there, and once the siding is snapped into place run a small bead of clear silicone caulk between the top piece of siding and the one under it. They will expand and contract together with the silicone allowing for slightly different rates of movement. If the top of the siding is loose in the "J" or the lock part is exposed and ugly, cut the bottom of a piece of siding to fill the gap and attach it similarly.
Phil