If you are simply transporting a laptop, fine, but my advice would be to avoid the hack jobs, I've gone through a few options. Seat post mounted racks and p-clips and QR lever mounted racks, all which ended up in the bin.
Both had ridiculous weight restrictions, ~10kg max weight and both rattled and rubbed profusely and worst of all I suffered from heel strike with every rotation of the cranks unless I pivoted the rack unnaturally, which unbalanced the bike. I couldn't do any serious shopping on commute home which would have been nice considering I owned a rack! Bear in mind my d-lock weighs about 1.5kg, the pannier bags another 0.5-.75kg, leaving me barely enough capacity to get a few cans of beer, spuds and onions in along side my work kit. Maybe I was unlucky and demanding too much, but it wasn't a solution for me.
Instead I opted for a second bike I built myself for which is designed for touring (extra length in seat/chain stays to prevent heel strike!) and I stopped bastardising my road racing bike with accessories it was never meant to carry. Aslo consider you might be putting strain on components that were not designed for the extra weight, its not unrealistic to suggest you shorten the life of your wheel spokes and carbon frame with the extra weight if it's a racing bike by design. If space isn't tight, I consider adding a second hand touring bike to your collection, complete with front and rear racks so you can balance loads. Otherwise, a laptop in a messenger bag should suffice!
I would question why one would commute on a carbon framed bike and add extra weight to it with a rack and lug cargo around with it, especially if you can have the option of having a second bike.