The rock guards are pretty much standard sizes anyway - it's just the mounting brackets that differ from one to another. The standard sizes are 96 x 16" and 96 x 20". Ultraguard offers a 94 x 16" model too.
Or order a custom made one here:
Motorhome Mud Flaps And RV Mudflaps
$429 for one that says Discovery. Or put your own name on it!
I would order one for the later model and assume I could adapt the mounts. The one on my 2004 American Coach is just slung from chains anyway, and it has an adjustable link. I've already adjusted mine to run a bit higher off the pavement than the factory set up.