Too funny. We also have had the same quandary re: shutting off the thru hull. We hate to remove all the cushions, slides the boards out, etc. every time we want to sail. Some have the belief to NOT shut the valves, we decided it was a good practice. So.....instead we decided to not go through the hassle and just have a jug of water by the toilet and use that for flushing. I LOVE the idea of cutting a hole in the boards. As mentioned by another post, I will first seek a template cover to put in there. I know you can get round covers in the plumbing department at Lowe's/HD so the hole to cut may end up being round, not square. A cover for your square cut hole may be easily found with a deck rail cap, size dependent.