Cage How to make this two story

So I've been trying to figure out how to add on a second story to this pen that is safe and secure so my three boys can have a little more room. Any ideas on how to do this that won't end up looking like a train wreck? I was thinking of just screwing four legs to the pen, sitting another slab of wood on top of those legs and screwing them in so they are solid then start building the second level with fleece and puppy pads and then grids around the outside so they can't fall off. To make the ramp I was thinking of cutting PVC pipe in half, adding some fleece and attaching it all in a ramp.