Firstly, welcome to the forum. Unfortunately your info above is incorrect.
I have roof racks rated at 100kg on my Dmax.
The factory roof rails on both the Dmax and the MUX have to mount to something. Isuzu don't make one roof for those models without roof rails and a seperate roof for the higher spec model with roof rails. That wouldn't be economical or practical. Isuzu make one roof for all models. And that roof has captive nuts to mount the roof rails too. If your model does not have rails, the captive nuts are still there under the rubber strip on the roof. All you need to do is cut the strip at the correct spot, dig out the silicon filler from the nut, and fit a mounting plate for your roof rails racks.
Super Cheap Auto sell Prorack bars which fit.
KB
Thanks for the welcome Kingbilly
Actually my info is correct as I was talking about the MU-X not the D-Max LS-U which as you state has mounting points under the ditch mold which you need to cut and is the one you are using I'm assuming.
As for the MU-X LS-T with rails you are correct again and I also said the same, as you state there is a place to mount a roof rack as the rails have to have some where to mount and any mounting kit will use this area to to hold the roof racks.
Our investigations into the lower spec MU-X LS-M without rails and the deep channels that run up either side the roof, there is no available mounting points on the roof or under the ditch mold that we have been able to find or Isuzu can tell us about, happy to be proven wrong if some one wants to rip their ditch mold out and have a look, photos would be nice too. It appears Isuzu did make 2 roofs for the different spec MU-X's vehicles. You could always use K323 as a permanent mount but it requires drilling 8 holes in your roof and putting some pop rivets in to hold the small plates.
Cheers Stephen