The size of your cage is fine. something to think about, is that you may want to go just a little thinner than 3 feet to clear doors. But if this isnt a problem, I would go 3 feet.
For plywood, you will probably need 2- 8 foot sheets, and I would use waferboard because it will probably cost about half what plywood will.
For the frames, I would use 2x2 instead of 2x4. They are plenty strong, and the cage will be much easier to move around, and lighter as well. for the doors, if you are going to have sliders, just get 2 peices of plexiglass that are both a few inches longer than half your hole. I use swinging doors on mine personnaly. Hope I helped out
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1.0 Albino Green Burm
0.2 Normal Burm
1.1 Java Retic