I just put one shelf in my wooden vivs - about 1/3 the length of the viv.
Aquarium sealant should work - you'd have to support the shelf with something while it dries though.
Alternatively, you could make a removable structure out of 3 pieces of contiboard - like a small table in the viv.
If it's an aquarium-type viv with a screen lid, make sure the lid is really secure as adding a shelf will give the snake a bit more leverage to push the lid up.
the viv is safe enough as its used to home the biggest boa i have ever seen and the glass is very thick so that wont be a problem its just im a wee bit thick at diy and i normally get a fella to do it for me cos if it goes wrong i cant really shout at myself can i just need some ideas and stuff thanks guys n gals and all the best for 2007xxxxx