Burmese Python Caging Question

For now, I keep my female albino Burm Lakshmi in a 4' x 2' x 17.5" expandable Boaphile plastics cage. She is about 6.5' long and growing ... and I do mean GROWING.

I'm thinking about buying an expansion for the cage. If I extend it 6' or 8' it will be either 10' x 2' x 17.5" or 12' x 2' x 17.5". While the length should be sufficient for years to come I have some concerns about the depth. She's not particularly heavy-bodied at present, but I know that as they get older Burms can gain girth as well as length.

Would I be better off biting the bullet right now and getting a custom-built cage for her in 8' x 4' x 2' or something similar? Or will an expanded version of the present cage do? As far as building my own goes, I am at two with woodworking: I'd probably have to hire someone else to do it. However, I'm not sure that the final cost would be that much greater than the cost of an expansion unit.... and unless her growth spurt takes her into 16' + territory I'd probably be able to use the cage for a good long while.