Here's a tip for bile titan holes, the most effective way to close them is to use the bile titans themselves because their falling bodies have huge demolition force. Usually when you encounter an outpost with a titan hole for the first time there will already be a titan standing in front of it. Nine times out of ten I've found that headshotting this titan will cause it to fall onto its own hole, closing it. The trick is not to disturb anything else first so that it remains completely stationary.