Well call me your Ishmael, my friends, because I've done it. I've found an under $50 maxi dress from Anthropologie.
With a long, sweeping skirt and full, roll-tab sleeves, it's the type of dress you can throw on for a quick trip to the grocery store, a date night with your husband, or a movie night with the girls. The full skirt provides ample coverage and feels like air against your skin (you'll also feel like you're floating on it, with how great you feel modestly styled in this dress!).
Currently, they have sizes S-L all available. Buy one for you, one for a friend, and reap the rewards of a modest summer dress!