Securing Nest Box Post

This is probably a dumb question, but I can't find an answer using the search function. I am doing my first woodduck nest box. The customer wants it on a post that will be secured to a base with artificial water around it. My question is how is the best way to secure the post. The box seems fairly heavy & the post will be about 3 ft tall. I have thought about doing some 2x4's on their edge & screwed to the bottom of the post in the shap of a tic tac toe board with the post in the middle. After that I was thinking about going with painted plywood on the top of the 2x4's & pour the water on top 1/2 to an 1 deep. Maybe finish it up with some moulding around the outside. What does everybody do in this case to make sure the post isn't wobbly & the base has enough weight not to let it tip over? I'm not too worried about this thing standing up to transporting it since the box will be able to be removed from the post.