smaller handlebar bags can be mounted directly to the handelbars without to much trouble, but that the size you want, the best bet for a handlebar bag is a rack mounted version. As Clifton pointed out, they sit much lower and tend to be more stable. They would work for your application, but they would leave an unsightly rack on your bike when the bag was removed.

It might be a good idea to have two seatpost setups. Buy a cheap aluminium one (think kalloy), attach a seat-post mounted rack, a trunk bag, and a cheap commute saddle. Save the carbon seatpost and fancier saddle for longer rides without a commute load.