Well those boxes says that if you want to put the slot at the bottom of the box like this one:
you should place the driver in the center of the box. Also it says that there will be midrange frequencies coming out of the port, so the box only be used to about 250 Hz.
Hornresp is very limited when it comes to reflex boxes, you can only place the port at the end of the box. But it can simulate what happens inside the box (limited to one dimension). It is also useful because you can export an akabak script from such a design so you don't have to make one from start.