Never actually understood why people use fake a lakes, all that crawling under a boat with a plunger seems hard work.
If you don't have a raw water strainer fitted then fit one like this
Then all you have to do to flush is un screw the lid and stick your hose pipe in it. Added bonus is that it will keep all the weed and stuff out your trans cooler.
If you don't fancy that then it takes less than 20 seconds to remove the raw water hose and then you can stick the end of the hose pipe in there.