Yeah inside the pot and just kind of bury as much of the pot into the substrate as you can to give it some stability. Give it a really good heavy watering/shower to flush out any kind of pesticides and whatnot the best you can before placing it in there. If he tramples it or knocks it over, oh well. Depending what kind of plants, they can be pretty forgiving as long as their root structure is good. Pothos and similar do really well for me in enclosures because when they break they just have 5 more new leaf shoots sprout in a couple days, not to mention being really tolerant of less light since BPs don't like bright lighting.