"I carry 2 1L platys in my backpack, but on my first backpacking trip, we decided to do a day hike and I had no way to carry water."
Take your Platy, turn it upside down, and with a HOT nail, melt a hole in each corner. If you want to be fancy, use some grosgrain webbing for a sling. Otherwise, simple, fat, cord will do.
And sling it over your shoulder.