I was facing this problem too. Used the messy concealers for a time, and let the topper hair hang longer at the sides, but it was an uphill battle.

Eventually changed to a full cap that covers temples, sideburns, the lot. Much more effective.

You can "sculpture" the full cap perimeter to your exact requirements in these areas, finely tapering the sideburns, shaping the temple points and trimming in more recession if need be. I generally do this very carefully using two mirrors and some small manicure sissors with the full cap on my head.