go with what you think is best if you are working large peices say a HUGE anvil a bathtub could work well personaly i am not makeing anvils or huge archetural peices so i go with a modifyied washtub forge
if you want to make a forge out of it ok but is i infer from your second post you want to make a bigger forge. the washtub forge is big and can be restricted to use less fuel when it is not needed the ramrod idea it a amasing idea and will make changeing the fire size relitively easy