I did this on a truck once, and it worked. It is not ideal, but it will work fine. BackRack makes brackets for their racks that have the necessary 3/4 inch hole for the NMO mounts. A 1/4 wave antenna would work fine, just make sure its at the top of the rack and in the clear. Anytime you get the antenna down below surrounding metal, performance is going to suffer. I'd suggest making sure the rack is well grounded to the sheet metal. Running a piece of braid from the rack to the sheet metal might help performance and make it easier to tune, might not, but it won't hurt.
Center of the truck roof is preferred, but I understand some don't want a hole punched in the roof of a brand new truck. Just make sure you add a disclaimer to it when you install the antenna. The owner/driver should not expect it to work as well as an antenna mounted on the truck roof. Range and performance may suffer a bit since the ground plane is not ideal at your frequency.
Down near the bottom of the page. Likely you could fabricate this on your own easier/cheaper, but at least this gives you a good idea. One thing to consider is that since it's likely going to be out on the edge, you need to make the mount strong enough for getting hit by a branch, or someone using it as a tie down point.