If I don’t have mini cupcake tins and I am 100% committed to making them, can I close them with toothpicks after filling? Any other suggestions?

Why not try to make a temporary mini-muffin pan by molding doubled or triple layers of foil over the back of an egg carton? Then remove the egg carton, and you should have cups that fit your need. You might need to make a couple of the molds, and bake on another baking sheet for support- but it should work okay.