You can perhaps find small muslin bags that are used by beer brewers to hold hops or hop pellets or you can simply add these spices to your secondary and taste after a few days and then each day after and so rack your mead off the spices when you think the right amount of flavor has been added. That said, one possible problem might be that you want to remove the cinnamon after X days and the oak after Y weeks.. So another option might be to make tinctures (using vodka) of the cinnamon, the oak and the vanilla and bench test to determine the total amount of each tincture you want to add. You could use a cup or less of vodka for each spice (and oak) and you might need to add about a tea spoon of each or thereabouts. It may take a couple of weeks or more to extract the flavors you want.
