I guess the question is how big of a flathead are you hoping for. I have set a couple myself but I rarely fish near the boxes. I don't want to run them off. I set the boxes out to draw the fish to the area that I fish but to answer your question. I used a 55 gal plastic drum and cut it in half to make two boxes. I have seen an increase in flathead activity since doing so too. Good luck with it. I think you can put anysize box there to draw big cats.
I have caught several pairs of 40's out of cast iron bathtubs our theroy is big holes produce big fish:wink:
