Hello CH! I'm interested to participate in your contest. It would be nice if you can #guaranteed the contest. Some of the designers feel confident to participate in contests that offer to protect the work by guaranteeing the final price.
You can use this following links to have an idea of what is a generic prefab logo: (beware! Because you might get something similar to these as well, happens almost every time)
And this one to check if the design you're thinking to choose has been used before:
Also, you could #sealed the contest to avoiding internal copying.
Now you can be sure which designer is giving you a design completely made by itself and you can feedback and rating to guide them get better. Some of the designers feel more confident to participate in contests that offer to protect the work by sealing it and by guaranteeing the final price.