I've installed two NT-30s within the last year but the exterior vent grill was slightly different than the orginal, so I had to drill new holes at the bottom. They had no problem gripping, obviously.
I just finished installing the second one last week.
I'm trying to remember what the original looked like....
Are the intake and exhaust tubes one piece? (On the new ones, the intake is comes separate from the exhaust and is mounted separately before installing grill with exhaust tube. (See pics below)
The 6 screws which hold the grill should grab the skin. (You may need larger diameter screws. I think the top ones may also grab the exhaust from the inside or it may be a center screw in the exhaust, like the new one (see pic)
Again, it was a year ago when I removed the old one so I really can't remember. I know that the original was similar, but not the same as the new one.
Not sure if this helps much.