Heya Astroboy!
I just joined the solo cup challenge and decided to use coco with a little perlite. Problem with this is I know I will have to water a lotttt once the roots will the cup but I find I have had better success and faster growth rate with coco/perlite vs soil. I would reccomend some holes in the bottom of the cup and around the bottom sides for aeration and to help with the roots. I also like this mix because I feed the same water each time opposed to a feed-water-water schedule, less water I have to mix. I think later on I will be adding something on top to keep moisture in a little more.
Cheers and good luck ! Will be looking out for your journal!
Happy farming