Ideally you want a center mounted 1/4 wave antenna whip with a ground plane. There are NMO mounts that are for thick fiberglass body vehicles like ambulances. If there is steel where the NMO mount will clamp on the inside, you can scrape the paint away and have an electrical connection. That should work OK. Check with a magnet through the headliner and see if the steel is right there.
If you cannot electrically ground the NMO mount I would suggest an aluminum disc equal to or greater than 1/4 wave in radius mounted and epoxied atop the fiberglass shell. You should predrill the disc and position it after drilling the roof. This will be an ideal ground plane. You can buy durable teflon aluminum pie pans at target or wall mart for a nice disc, though you might need to trim the lip, and rough up the Teflon on the side to be epoxied. I would attach a grounding wire to the underside of the NMO mount in this case and ground it to the frame as close as possible. This because you don't want lightning or a downed power line entering the cab by this "floating" antenna mount.
If the fiberglass is really two inches thick, and you are using UHF, an external ground plane still might be required for a good VSWR and pattern . It wouldn't hurt to go the extra step if it is really that thick.
There are antennas sold as not requiring a ground plane, they are 5/8 wave, but not ideal as the coax is likely to radiate RF into the cabin and other electronics. Also they are tall and and more likely to snag and break if you hit some tree limbs.
Then there is this, not same model, but another idea. However the signal will tend to be unidirectional, meaning it will be weaker on the corner it is mounted.
https://m.youtube.com/watch?v=YMm9eY-xgOQ
Whatever you do, before drilling, be sure you are not cutting structural members as that cab is your rollcage.
Sent from my SM-T350 using Tapatalk