Wood Stain;
The stain will only minimally protect the wood against the damp/hot environment (which is present inside a cage). It does soak into wood, giving it
some resistance to water. However, it does
not provide the necessary protection. You'll have to apply lacquer to actually seal the wood.
Personally, what I've always done is...cut out thin sheets of plexi and attach them on the inside (sealing the corners with caulking). Its less messy that way
. I have no idea if the chemicals/fumes from lacquer will negatively impact your animal. I would assume no (once it dries up)...however I have never researched that, so I cant give you a definitive "yes" or "no" answer.
Heating;
Unless you have cut outs on the top of your cage, then you'll probably want to stay clear of using ceramic bulbs altogether. Keeping them on the inside of the enclosure could cause problems. You don't want your retic to be in direct contact with such heating elements...they can cause some really nasty burns. Radiant heat panels ( such as these...
http://www.reptilebasics.com/rbi-radiant-heat_panels/ ) would be a better choice.
Glass Front;
(
I know this wasn't solicited but I'll talk about it anyway)
Make sure the glass is
very strong (taking into account the large and powerful animal that you'll be housing). Having a snake breaking through the glass can cause it potentially life threatening injuries. I've always used thick plexi doors for any caging that I've built.
Good luck on your project and update us by posting some pictures when you're finished.