Here's a couple of certificates and a poster I've knocked up for you to use as you want for your participants.

 

Certificate jpg Certificate pdf Poster pdf

And here are the originals in Microsoft Publisher format if you want to download and add dates or do something more fancy...

Poster: 

https://drive.google.com/file/d/1a182WQwYcBqVy0Hr8swgRCSh0wZsOiDh/view?usp=sharing

Certificate:

https://drive.google.com/file/d/1trgH1duEcWc6CeiPq-m-GchYHpIcMI9k/view?usp=sharing