I was estimating 20x8x4, 4 being the max height of the waterline. Still calculates out to 23lbs., higher than my first estimate. Thanks!

As far as the baffle, if I do it, I would put a half hole in the very bottom where it hits the hull. This would still stop the main surge of water on a fast lean (like setting the hook on a tarpon!) and would allow the water to stay at the same level and drain using one drain hole.

With a 20" wide square box 8" long and 8" deep you will have
: 45lbs of water.
: 20 X 8 X 8 = 1280 cubic inches. Careful the volume adds up very fast!
: 1728 cubic inches = a cubic foot which is 62 lbs of fresh water.