This is the problem that is answered by a box of the 816 or 817 type. Depending on your crossover point you need that type of box, or you need to see the 515b as a compression driver which will need a back chamber of the correct size.
I think I did not quite take your meaning because I more or less automatically assumed the lower portion of that box does not have sufficient frontal area or interior space for an 18" woofer, which would be my choice.
You will wonder this more than once in the course of designing and building a diy speaker. I think you are right to look for some tracks in the snow. I haven't made any in this part of the woods.