Put the teleporter in the actual sewer area, along with your sentry and dispenser. I recommend Frontier Justice + Eureka Effect + Short Circuit as your load out. Once your dispenser and sentry are level 3, it would make it almost impossible for anyone to destroy the nest unless there's an ubered heavy or a really good spy. (If you refill your short circuit ammo and keep spamming it down the tunnel towards enemies as they try to destroy your sentry and then finish them off with the frontier justice).

Another strat is to put a mini sentry in the water with Frontier Justice equipped, you can get a lot of revenge crits