any driver can't just be plopped into any enclosure. the driver parameters must be matched to the enclosure.
you are in luck though because sonosubs are as easy to design as ported enclosures.
for the lowest cost possible, here is what to do:
1 piece of 18" diameter sonotube that is 48" long
1 piece of 6" diameter sonotube that is 24" long
2 pieces of wood 3/4" thick, circular, that fit exactly into each end of the 18" sonotube
1 piece of wood, circular, a little larger than 18" for the top
glue the 18" endpieces into the sonotube. make sure they are air tight.
mount the driver on one end and the port on the other end.
add some 1" dowels that are at least 2" long for the feet on the bottom and the feet on the top.
to the top "feet" add another round piece of wood a little larger than 18". this is what your dad can put the plant pot on.
this design will work with the driver and amp that you mention and will have a little over 6 cubic feet interior volume and about a 21hz tune.
finish it however you like.