I would cut back on her feeding if she is that large at that age.
My adult boas are all fine in 48" x 30" x 18" enclosures but they weren't as large as yours at only three years old either. Yours should be fine in a six foot wide enclosure. I'd go ahead and make it 30" deep and 18" high.
I would make it out of 3/4" thick PVC. Yes it's expensive but given that your boa could live for 20 more years it will more than pay for itself in ease of cleaning, maintenance, energy required to heat it, etc.