Cute idea... good way to get them to learn square roots without them knowing it because they know the numbers under the post its! It is very inexpensive and not at all time consuming to make. The only problem I can think of with this would be the postit notes loosing their ability to stick onto the clock's face.
Cubed Root: Find the prime factors of the number under the radical; look for groups of 3 of the same number, like 3 - 2's (cubed root of 8) to take out of the radical; if negative, can factor out the -1
Standardized tests require students to work through problems at lightning speeds and square roots are some of the trickiest answers to find for a student just learning about them. This video gives hints and tips to make the process cut as many corners as necessary to find the correct answer.
CCSS-8.EE.2: Evaluate square roots cube roots. There are 3 different cards to sort: 1) square roots, 2) cube roots, 3) mixed roots. Students determine which square/cube roots are "like radicals", those w/the same index radicand. It could also be used as group competitions. :)