The big thing wrong with your dimensions is the dimensions themselves. At Home Depot, the common dimensions of wood they're offering is:
8'x4', 4'x2', 2'x2'. Some boards you may get 12-16' long, but that's difficult to handle. Your major dimensions should equal or evenly divide into those numbers. Don't forget the kerf of a tablesaw blade is around 1/8" so you need to factor that out for every cut- one 8' board does not equal two 4' pieces, so planning your cuts ahead of time using most of the board but not quite all of it tends to work well.
Unless you're trying to fit into an odd sized space or do somethign compelling, there's not a whole lot of reason to deviate from the normal measures. You'll be wasting material and undergoing more cutting. More cuts introduce more errors, etc.
Attached is a photo of some containers I started early yesterday morning and I'm now taking a break from today- all that's left is to do the door, interior treatment, and lid. I don't even have material for sealing the interior, so I may finish the doors this weekend and treat them next. These are 4'x2'x18" interior, 20" height total. I'd have to estimate weight at about 20 pounds each. Very sturdy, light, and real inexpensive to make.
My floors are 1/4" oak or birch plywood, rear and top is 1/4" particleboard, all framing is done with 1x2s (which are really 1.5x.75). The floor rests over five supporting boards and it's got the frame on top providing stability, so despite being .25" thick, it's exceedingly stiff. The sides are .1" acrylic windows, the same material I'll be using inside of the doors.
All assembly is glued and nailed (or stapled) in at least two dimensions. What isn't there yet is my "no jam" door hinge design that basically consists of 1.25" hardwood dowels pivoting inside PVC pipe caps. SHould be highly resistant to the geyri kicking sand into it and shouldn't scare the animals when I open it 
I'll be moving a group of four ornates, three geyri, and three macfads into this once it's done. Probably will leave the other one vacant and move some other animals around until somthing catches my eye enough to populate it. Got another pair of containers to do after these to match up with my 3'x28"x18" Visions so I can just stack them together.
Now, if you wanted to do a "double height" kind of unit that wouldn't otherwise make it through a door, you could do modules in a similar fashion to these but omit the interposing ceiling and floor.

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2.3.0 uromastyx geyri (Saharan/Nigerian)
0.0.4 uromastyx hardwickii (Indian)
3.6.0 uromastyx macfadyeni (Somalian)
1.2.0 uromastyx ocellata (Sudanese)
1.3.0 uromastyx ornata (Ornate)