Well, you can get a 3 tub Cb70 rack from Reptile Basics. Your ball will be most likely set for life in it and your boa will be good to go in there until he hits the 5' mark or so. I used a tank for a little over a month and I think a rack is the way to go, much easier temp control and I never had a bad shed ever in the last year that I had my snakes in a rack.
I don't think a rack is waste of money especially if you plan on getting a few more balls. The boa however will obviously need something larger later down the road.
Also, the rack is made to fit which ever size tub you plan on using, once/if your snake out grows it you need a new rack.
http://www.reptilebasics.com/For-Lar...p-1-c-305.html