Doors and Windows - Hollow Core Doors

What is the thinnest Hollow Core Doors that are made? I'd like to "make" some doors for an under the stairwell closet. It has two openings. Both openings are essentially rectangles that are angled downward on top to follow the contour of the stairwell i.e basically a rectangle whose top is a triangle's hypotenuse. I was considering using various types of plywood, MDF, particle board, etc - the problem being is that one of the doors is about 70" in height& in order to have a door that long I feel the wood needs to be at least 5/8 - 3/4" thick so it won't warp/bow. Unfortunately wood this size that is that thick is fairly heavy. The closet is one in which kids toys are stored so I'd like my kids to be able to get in & out easily. I guess I could theorectically use a typical hollow core plain door & try cutting it to size, the problem being is trying to actually customize it to the right size & the fact that the standard 1 1/2" thickness of most doors is thicker than I'd like. So either I need thinner hollow core doors to work with or lighter weight wood. Thanks

Standard interior hollow core doors are 1 3/8 thick. Any good supplier could cut the angle on it and reblock the cut side. You could do this yourself if you have some woodworking tools.