You're having an argument with physics, and physics always always wins.
IMO there is no way to carry a nikon slr with the 18-200 on a bouncing mtn bike at 10-20 mph, on your person. To keep it from destroying itself or injuring you, the case would be 1) bulky 2) likely affect (negatively) your control of the bike. I agree with marksmith. If you are going to carry it on your person (somehow) get a smaller camera or just use your cell phone.
I travel a lot by motorcycle. On the back I have a 1550 Pelican case with cutouts for a d810 and three lenses. It is secure, but if I see something I want to shoot it means, stopping, taking off helmet, gloves, getting off the bike, opening the case, taking the pic then loading back up. Over the years there were many pictures I missed and I still regret not stopping etc...soooooo..... I got a Sony RX100mk2 that I carry in a jacket pocket, or my tank bag, and even sometimes that is a bit of a hassle, so I use my Note 4 as a backup to the backup. But they ALL get used.
That said, I also shoot night time Mardi Gras parades, and I ride my bicycle to them, carrying the d810 with a 24-70. I have soft panniers on the bike so I use a Think Tank bag/case to hold it and put it in a pannier...works fine, but that is all I am carrying. And when I stop to shoot, I am in one location for the parade, then pack up and ride home. ON the motorcycle I am carrying clothes, shaving stuff, electronics, etc, IOW I am loaded. And stopping intermittantly
But for the bicycle the Think Tank case, in a rear pannier works great-again, though I am riding to a single destination, shooting and then departing, so your needs sound more like my motorcycle experience than my bicycle one.
Think Tank - Best camera bags, shoulder bags, backpacks, and rollers