As far as i know Front Runner are the approved supplies of roofracks specifically designed for this type of vehicle.
My only advice is to make sure your RTT doesn’t overhang too much on the bonnet front, as Jimny’s are great fun off-road they are more vertical than horizontal. which means air flow driving at 120 on the open road may catch an updrift. and then you could be in trouble!
however, if you’re driving at reasonable speeds you should have an absolute blast at any 4×4 track with one 😉