Cover them up!
My first thought is, that whatever you come up with, they should be covered. And, hopefully with something like one of those sliding "tonneau" covers. I think that's what they're called.
I could see a piece of Plywood, say, 24 or 30 inches wide, with the appropriate length, and with uprights carved out to cradle the MD's. You could line the cradles with foam or felt and use Velcro straps to secure them. Of course this would then be under some kind of cover.
You can count on one thing. If they are visible, some worthless POS will break in and steal them.
Good luck,
EKW