Personally if your snakes are 18"-21", I would not build them an enclosure at all. It's not worth the time, effort or $$.

At those lengths I would just keep them in 12qt sterilite type tubs ($2.00) water bowl, hide and substrate. When the time comes (about 6-8 months of age) move them up to 27qt tubs ($3.00), once they outgrow the 27qt tub (about 18 months of age) move them to a CB110 tub ($12.00). A male boa can live in these tubs all their lives, females will need their own 4X2 enclosure at about 5ft. At this time you can start planning your permanent enclosure for your female, which I recommend be about 4X2 and if you plan on growing her to over 7ft, get a 6X2.