Hey Tetsuya, I have an idea to help prevent any cheating.
Instead of burying bells, why don't you bury specific type of items to represent the bells?
For example, you could do gems. Emerald = 1,000, Sapphire = 10,000, etc. Or stationary, seashells, anything that has some variants that are easy to find/cheap.
You wouldn't want to announce what the items are until everyone is at the contest of course, but I think it could work!