These seem perfect...but I have been doing the exact same thing with an Old Navy womens' Tall Tank. It is so low cut that I could never wear it just by itself, but I have been wearing one under my t shirts now for the entire winter and it covers my tummy totally but I can nurse via the low cut neck/chest area. Much cheaper, same deal.
I know women have been dissatisfied for some time with the nursing clothes because they are all about hiding the boob and not the often disturbing (to the Mama) postpartum midsection. Many of you wrote to me a while back about little home made tank tops and underthingies you have concocted to help with this. I am here to vouch for the Old Navy Tank top in tall. They really are tall. You might not even need tall. But I do.