If you are young and fit, you can soon get the hang of climbing on the roof by opening a 2nd row door, and then step up from the side step, a 2nd row seat and then on to the arm rest of the door and then onto the roof.
In Morocco we had two rood tents and I had to get up inbetween them to set them up, I was a little stow at first, but after the first few days was up and down there like a monkey ;-)
If you aren't comfortable with that I would aim to try to make something up yourself, ideally something like a cheap aluminium ladder from a DIY store that can live under the roof rack and then be unclipped, pulled down to rest on the side of the vehicle (with rubber feet) that you can then get to using the side step.
I wouldn't have something permanent there, it is likely to get knocked off.
- Specialising in Land Cruiser preparation and servicing.