You are a little more limited on choice with the defender wheelbase being 324mm whereas the majority cater for a 313mm wheelbase a la scx10, trx-4 sport, enduro etc.
Proline do a couple specific for the defender if you don't mind painting,and killerbody do some pre painted specific also.
You could always try a shorter 'base shell and elongate the wheel arches a little, tho that sometimes looks a little odd.
As for the inner arches Traxxas do a couple of sets that secure to the chassis if you really want to keep the mud etc out. But if you did that it wouldn't be a quick swap between shells. Probably best off running the beater shell without any tbh anyway, less for tyres to rub on during extreme terrain