Here are a few suggestions.
The design you looks great.
Here are some things that came to my mind when I looked at your drawing:
1) you could use peg-board (a sort of formica with 1/8th inch holes drilled throughout as a back. This would be a 'quick and dirty' way of getting ventilation.
2) hinge a one-piece door at the bottom with a piano hinge (available at Home Depot, or Lowes) and use a bolt latch, or two, at the top to keep the door closed.
3) extend the cage at the bottom to include a shelf area--much like you see in a rack system--where you can slide a large Sterilite, or Rubbermaid, blanket box. Drill, or cut, a large hole in the bottom of the cage, so that your snake can go through the hole and down into the blanket box. That could be its hiding place. Also, you could keep some damp, long-fibered Sphagnum moss in there for it to bed down on.
4) make the bottom of your cage out of a piece of (Dang, I forgot the name...to many altered states as a youth, no doubt.) that partical board stuff covered in masonite.
These are just some of the things that came to me when I looked at your drawing.