On my old Rig the built in ladder got broken from the former owner backing into something. They did a good fix, but i never really felt it was sturdy. Before I would try and retrofit one, I would go with the collapsible ladder. Their great for around the house to and very compact. If you install a ladder, you can't be quite sure what you'll drill into. Plus if there is a curvature to the roof, the part that connects will need to be offset a little. I would be surprised if you found a proper fit, but I wish you luck.