What a find! You're so lucky!
I think the little shells are cute. I agree that blue tac or something like that would be a good way to anchor them.
I'm very very limited in what I can pay for art supplies, so I love a bargain. I was wanting to make myself a little travel kit using half pans in a breath mint tin but I don't have any of the little magnetic dots and can't buy any. But the other day, they passed around some new phone books and on the front there was a business card that was magnetic to stick on the fridge. Voila! Magnetic stuff to make the little watercolor travel box.
It's wonderful when we keep our eyes peeled and our imaginations in gear isn't it? Who would have thought to look at Goodwill for watercolors? You did and it paid off big time. Congratulations! Let us know how you like the watercolors in paintings.