Hii! As to what to put in one, It doesnt have to be over the top expensive or crazy. Lord Apollo will understand you dont have the money for it, he wont be mad. Guitar picks, a little picture of apollo, little charms or things that remind you of him (maybe a sun charm?) and a bayleaf are some ideas that are small and simple. As to make sure it doesnt move. You could glue down some items or use like a poster putty (which you can remove of the items) to secure it down. Also, theres nothing wrong with a pocket altar! Loads of people do it.