Functional objects like work benches, torches, smelters, fermenters, campfires, portals, and so on will create a 20 meter no-spawn zone. (purely structure objects like walls, doors, roofs, do not). So if you make sure the islands land is fully covered by the no spawn zones it'll be safe. If you are unsure its fully covered go to various ends of the island and make a lot of noise to trigger spawns. If nothing shows up you are good. If it does figure out where it spawned and fix the gap. Note the game tries to avoid spawning in your direct unobstructed view so look away while testing.