I agree about the sizing for large constrictors, or any cage for that matter. The only burm cage I built that was 4' deep was a walk in sized enclosure, not something I had to stick my torso into to get the snake.
The depth to height ratio can vary depending on a few factors but in general don't design a cage you have to lean your torso into.
Also, I am not a fan of cages built from 2x2 framing with sheeting over that, especially 1/8" plastic sheeting. 2x2 framing is very likely to warp and twist over time and is hard to join accurately. If you have some 2x4's that have been dried for at least a year and can mill your own 2x2's that might be a different story. If you just plan to go to the store and pick out the straightest 2x2'2 you can find you're asking for trouble.
I would build a basic carcass out of 1/2" wood sheet material at a minimum. Maybe even two 6' cages that can be joined together with bolts. The line the cage interior with your free plastic.
If you're worried about weight make large cutouts in the back sides, top and even the bottom before laminating the plastic to the interior. This saves a significant amount of weight and little strength is lost.
Bighurt and I talked a poster last year through the process of building two 5'x2.5'x2.5' cages that could be bolted together to make one 10' long cage. This was accomplished with the super high quality baltic birch plywood that comes in 5x5 sheets.
He did use an interior frame which I don't like but he did end up with what I consider to be about the ideal burmese cage.
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Current snakes:
0.0.1 Gonyosoma oxycephala - Java locale (green)
0.2 Gonyosoma oxycephala - Malaysian locale (green)
1.2 Gonyosoma oxycephala - Java local (green)
2.2 Gonyosoma janseni - Seleyar locale (all black)
1.2 Gonyosoma janseni - Celebes locale (Black & Tan)