So here are my suggestions.
Below I have compiled a list of all participants along with their reputation and entry post payout. The purpose behind putting reputation and post payout along with contestants is allow
Why reputation score between 45 and 55
In my experience the steemians with reputation score between 45 and 55 are the struggling ones. They are usually competent and genuine enough to achieve that reputation score, but are not famous like 60+ reputation steemians to earn as much as their content deserve . So I feel when choosing a winner for his Behind the Mystery Door Contest,
I am in no way trying to undermine 60+ reputation steemians but I feel the steemians between 45-55 reputation score are the most deserving ones. So at least one winner among three, should must fall in this class. That is why I have also put reputation score in table below.
These are only my suggestions and I leave the rest at your better judgment
Hope 
finds my suggestions practical and I earn my bounty:)
Over and out!!!