If a cubit is 18 inches and a span is 9 inches and a hand is 4 1/2 inches and you built a metal bowl with an outside diameter of 10 cubits and a thickness of one "hand", and you measured the inside circumference of the bowl, how many cubits would it be?
You're being unduly conservative. Make the thickness equal to 0.225352 cubits, and you can have the inner circumference exactly 30 cubits.
So, since we're fudging, make the day equal to a few billion years, and Genesis won't conflict nearly so badly with science.