Circular pads would be kewl. Keep in mind that a 5' x 15' x 5" thick pad will take approx 1.2 cubic yards of concrete and a 15' dia circle 5" thick will take 2.7 cubic yards of concrete. If you decide to go circular, I would suggest you go no smaller tha 12' diameter, which if 5" thick, would take 1.7 cubic yards of concrete.
Just offering up my 2 cents worth on the subject here.... I would recommend getting the form lumber (2x6) to do 6 boxes. Do the excavation/fill, set the forms, making sure they are well oiled, get the subbase (earth or gravel) compacted, then pour out the 6. Wait a day or two, strip the forms and repeat until all 18 are done. Doing the work this way keeps it from being too overwhelming, and keeps you from spending a huge amount on form lumber.