Can't comment on building, as I've never had the materials, so I'll offer advice on height instead.
Figure out your current limit for vaulting height (I'll guess chest/nipple height). Then, build the box to be *higher* than this max, say chin height. That way, you'll have "growing room" in your box, rather like buying expensive clothes whilst growing up. As you won't want to waste all the effort building something you can't vault, you'll be motivated to increase your vault power
in addition, you'll learn the pop vault to help you in the meantime haha
not sure if I'm making sense,