Good luck mounting that
4.5kg would be great.. 9kg maybe impractical. You maybe better with 2x 2.5kg ones behind the stone guard.. One each side... May allow better access to the unit if theres a fire on one side.
Also if one decides to fail, you have a backup.
Your other 1/2 can also attack the fire from another angle with the second unit.
If your not sure of things with extinguishers (they are really easy), go see your local RFS/CFA station, they are usually (or were when I was in it) only too willing to blow a few units to demo to the local people.
Remember if you have Powder/Dry Chem extinguishers, even the 1kg jobs, you need to turn them upside down and shake them round a bit in your hands
As on rough roads the powder will just pack down like concrete inside the bottom and it wont work when you need it.
Do it every couple of days while your on the road... takes 30 seconds. IF you dont do it, you may as well have nothing.
Pick the one you think will be best for you...