How do you maintain good posture when working on things on the floor?
Don't spend more than 15 minutes in that position at a time. Get up, stretch your back, take a drink of water. Stretch forward, backwards, side-to-side, and twist both ways. Roll your shoulders forward and back. Stretch neck in a similar pattern. Be gentle to yourself.

Also, you can do some back-strengthening exercises (when you've recovered from this project) to get you ready for next time. See https://fitness.stackexchange.com/q/153/234