I know you didn't ask this, but it is my FIRM belief that a person your age should not own a burm...but I will bite my lip and hold my thoughts....sort of.
A 55 gallon tank, in my opinion, is not a suitable nor responsible way of keeping a burm, even a 5' burm. A 3-4' burm possesses a lot of strength and will have no problem lifting off a screen top (even it you put weights on the corners). There are MANY other probems with glass aquaria such as the open top that makes heating the cage sufficiently very, very diffuclt (but not impossible). If you can't afford proper housing, I would recommend buying the cage first and then save up for a burm....if you can't afford a good cage (like a neodesha) how are you going to afford your first vet bill?...it will happen. I don't mean to lecture but it doesn't sound like you are quite ready to take on this resposibility; and it is a HUGE responsibility.
In terms of an optimal cage for an adult, an 8' neodesha will suffice for an average size male or small female but you may have to go bigger if your burm ends up to be a fairly large adult (we keep our large burms in a cage that measures 12' x 4' x 3').
This isn't meant to cut you down in any way. I have several very talented 15 year olds that work for me at my wildlife center.....but have you thought about what you are going to do with this snake should you go to college? Who else is going to help you handle this snake? Does everyone in your family share your zeal for reptiles? Do your parents know exactly what they are getting themselves into? Believe, I made many mistakes at your age and with age comes wisdom; sorry if this sounds harsh.