Then you should not be getting a boa until you can afford to properly take care of it. The snake could outgrow the 20L in less than a year and then you'll be back to square one, only with a 3ft+ rapidly growing boa. And its not like a properly set up 20L will be cheap either...
For multiple animals a rack setup is the way to go anyways. Tanks are probably the worst. I have 1 tank and I will never house a snake in one again after I move her out into the new rack I just built.