I purchased a couple of inexpensive, extendable clothes bars, covered them with plumbing insulation to protect the rods, then wrapped an elastic cord from one end to the other. I use strong rubber bands to keep the end hooks of the bars secured to the hangers on the bulkhead of the vehicle. Excursions and Surburbans have four of these hangers with two just up and behind the front seats and two more in the rear of the vehicle.
I've had this type of set-up for nearly fifteen years and it has worked well for me. I don't know how much room (length) you have in your Discovery, but the interior of our Excursion accommodates all of my rods.
When using the Xterra I use a case that permits me to separate a rod into two pieces and leave the reel and line intact. This is not as convenient as the clothes rods and is more expensive.
I've experimented with the clothes-rod method in the Xterra where I separate the rod into two pieces, leave reel and line on, and wrap a bit of Velcro around the pieces. This is okay but not as tidy as using the case.
On short trips, i.e. moving upstream or downstream for just a mile or two I've used the clamps one can purchase from you favorite angler's store that are magnetized and affix to the exterior body of the vehicle. I've transported them reel-forward a few times and so far have not broken or lost a rod, but my vehicle speed probably hasn't exceeded 20 mph.
Allen R