Why do you want to use plywood?
The only two reasons I can think of are weight and/or appearance. I assume it's not cost since most plywood of good enough quality for a rack is more expensive than melamine and then still needs to be sealed somehow. Even then the boxes won't slide as smoothly as on melamine.
Let me know. There are ways to make a rack that is lighter and still attractive if that's what you want. But I won't get into that until I know what you want.
Regardless, plywood can be used but make sure to buy a nicer product. I have used Home Improvement Center quality birch laminated and would consider that to be a minimum.
Other products are going to be much to likely to warp which makes for a poor functioning rack. But you might find something cheaper that will work.
Of special concern is how the rack will work after shrinking a bit in the typical warm snake room. It might be best to allow the already-cut material to sit in your snake room for a couple of days before assembling the rack. I made this mistake once. I purchased a sheet of birch laminated plywood from a lumber yard with an unheated warehouse. I cut and asembled the rack that day. Once it was in my 80* snake room for a week the levels started to seize up. I took it apart and reassembled it and it worked fine for years after that.
Regarding thickness, 3/4" for the shelves and probably as thin as 1/4" for the sides.
Regarding ventilation, holes drilled near the front of the boxes is typicall considered enough.
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Current snakes:
0.2 Gonyosoma oxycephala - (Silver/Yellow)
3.3 Gonyosoma oxycephala - (Green)
2.1 Gonyosoma janseni - (Black)
3.3 Gonyosoma janseni - (Black & Tan)
