A good way to dry tomato, cantaloupe, and other seeds is to put them on blotting paper. They will quickly dry in this manner and will not become moldy, as the blotting paper soaks up the moisture.
I can recommend this method as I do it all the time and use organic paper towels so that they have no harmful residue and break down naturally so when it comes to planting out the seeds I just cut the paper with seeds on and place into the dirt. So easy.