I would probably have a good mix of open-ended and multiple choice questions, like some other replies suggested. Just be sure to keep in mind that this is a) a trial event and b) an invitational, and therefore the test shouldn't be crazy difficult. If you know other people doing this event, you could have someone else look over the test and give feedback on it.
(Possible) 2019-2020 Events (C): Designer Genes, Forensics, Wright Stuff, Write It Do It
2018-19 Events (C): Designer Genes, Fossils, Write It Do It (Mousetrap Vehicle and Experimental Design @ Regionals)
2017-18 Events (B): Rocks and Minerals, Wright Stuff, Write It Do It
I edit the wiki sometimes.