Hello Rvl,
I have done the exact thing you want to do, however my roll cage manufacturer PPCages in the UK make some clamps which I made an adaptor to fit.
My roll bar also sits below the roof line allowing me to set my final height as low as possible.
What I would do looking at yours is make some square bar to slot into the rooftent mounting channels, drill and tap 2 holes I used M8 and make 4 of these for each mounting point. Then make a nice strong strap to wrap around the cage and bolt to the channel inserts you made. Does this make sense?
Finally, just my own personal view I think a medium grand tour is rather large for a jimny? I have a medium and it only just fits my Defender90. Nice choice in tent though
Also, we have hired this car from Bobo campers this August in Namibia, the size Maggiolina fitted is a small.
Steve