You could use a func_plat, but I don't like those too much. Instead, use a func_door and change the lip and angle. You'll want to do the following to find the value for the lip: how high you want it - height of the elevator, then take the opposite (negative) of that.
Example: Func_door is 8 units high.
You want the elevator to go 800 units up.
800-8 = 792.
So you would have this: key: lip, Value:-792
Then make the angle -1 (for up) and put it on the bottom of where you want it to be. I don't remember what down is, but I think it's =2 or 0.
Hope this helps.