Well that's very big. So structural, did you mean 70 high by 48wide? Why so wide and not high?
For that, I would use PVC 4x8 sheets, or AC plywood. The price double ramps up there. And your proposed dimensions are good, as it will use almost a full sheet for the back panel, and 1/4 a sheet for the sides. If you flipped it, 70 tall, it would use full sheet back and half sheet sides.
So your looking about 50 a board for AC ply, and 100 for PVC. You would need 2 for the main frame. Then you could strengthen the top with 1x2s, and the bottom with your soil bin (assuming soil bin).
Then your doors made from 1x2s (assuming screen doors) or decorative trim, (alot of those have a good notch for stapling screen down) or you could cut grooves for it and spline into the frame, or just staple it on.
Unless you want solid doors, in which case you could do sliding glass, cut grooves for the glass in, or attach it to the back. You use Plexi, you can screw it in, and likely should as it will get scracthed and need replaced fairly often.
No matter what, Doors on a cage that large are going to be a pain.
That is less like building this threads, and more like building a cabinet.
This vid does it with PVC, wood be the same way though.
Ignore the door parts, that's a dumb way to do a door.
If you do go wood, go with Douglas Fir, it barely cost more than Pine, is much more rigid, and harder, and is not nearly as Poisionous to reptiles. It's still a soft wood, though on the harder side of softwoods. A 4x8 AC grade is like 40-50 for 3/4 inch.
Doing quick math... This would all add up rapidly. Maybe I should just get a second cage to attach to his current one lol. I think in the end I would be at just about the same price. LOL